cphb/luku09.tex

\chapter{Range queries}

\index{range query}
\index{sum query}
\index{minimum query}
\index{maximum query}

A \key{range query} asks to calculate some information
about the elements in a given range of an array.
Typical range queries are:
\begin{itemize}
\item \key{sum query}: calculate the sum of elements in a range
\item \key{minimum query}: find the smallest element in a range
\item \key{maximum query}: find the largest element in a range
\end{itemize}
For example, consider the range $[4,7]$ in the following array:
\begin{center}
\begin{tikzpicture}[scale=0.7]
\fill[color=lightgray] (3,0) rectangle (7,1);
\draw (0,0) grid (8,1);

\node at (0.5,0.5) {$1$};
\node at (1.5,0.5) {$3$};
\node at (2.5,0.5) {$8$};
\node at (3.5,0.5) {$4$};
\node at (4.5,0.5) {$6$};
\node at (5.5,0.5) {$1$};
\node at (6.5,0.5) {$3$};
\node at (7.5,0.5) {$4$};

\footnotesize
\node at (0.5,1.4) {$1$};
\node at (1.5,1.4) {$2$};
\node at (2.5,1.4) {$3$};
\node at (3.5,1.4) {$4$};
\node at (4.5,1.4) {$5$};
\node at (5.5,1.4) {$6$};
\node at (6.5,1.4) {$7$};
\node at (7.5,1.4) {$8$};
\end{tikzpicture}
\end{center}
In this range, the sum of elements is $4+6+1+3=16$,
the minimum element is 1 and the maximum element is 6.


An easy way to process range queries is
to go through all the elements in the range.
For example, we can calculate the sum
in a range $[a,b]$ as follows:

\begin{lstlisting}
int sum(int a, int b) {
    int s = 0;
    for (int i = a; i <= b; i++) {
        s += t[i];
    }
    return s;
}
\end{lstlisting}

The above function works in $O(n)$ time.
However, if the array is large and there are several queries,
such an approach is slow.
In this chapter, we will learn how
range queries can be processed much more efficiently.

\section{Static array queries}

We first focus on a simple situation where
the array is \key{static}, i.e.,
the elements never change between the queries.
In this case, it suffices to preprocess the
array and construct
a data structure that can be used for
finding the answer for
any possible range query efficiently.

\subsubsection{Sum query}

\index{prefix sum array}

Sum queries can be processed efficiently
by constructing a \key{sum array}
that contains the sum of elements in the range $[1,k]$
for each $k=1,2,\ldots,n$.
Using the sum array, the sum of elements in
any range $[a,b]$ of the original array can
be calculated in $O(1)$ time.

For example, for the array
\begin{center}
\begin{tikzpicture}[scale=0.7]
%\fill[color=lightgray] (3,0) rectangle (7,1);
\draw (0,0) grid (8,1);

\node at (0.5,0.5) {$1$};
\node at (1.5,0.5) {$3$};
\node at (2.5,0.5) {$4$};
\node at (3.5,0.5) {$8$};
\node at (4.5,0.5) {$6$};
\node at (5.5,0.5) {$1$};
\node at (6.5,0.5) {$4$};
\node at (7.5,0.5) {$2$};

\footnotesize
\node at (0.5,1.4) {$1$};
\node at (1.5,1.4) {$2$};
\node at (2.5,1.4) {$3$};
\node at (3.5,1.4) {$4$};
\node at (4.5,1.4) {$5$};
\node at (5.5,1.4) {$6$};
\node at (6.5,1.4) {$7$};
\node at (7.5,1.4) {$8$};
\end{tikzpicture}
\end{center}
the corresponding sum array is as follows:
\begin{center}
\begin{tikzpicture}[scale=0.7]
%\fill[color=lightgray] (3,0) rectangle (7,1);
\draw (0,0) grid (8,1);

\node at (0.5,0.5) {$1$};
\node at (1.5,0.5) {$4$};
\node at (2.5,0.5) {$8$};
\node at (3.5,0.5) {$16$};
\node at (4.5,0.5) {$22$};
\node at (5.5,0.5) {$23$};
\node at (6.5,0.5) {$27$};
\node at (7.5,0.5) {$29$};


\footnotesize
\node at (0.5,1.4) {$1$};
\node at (1.5,1.4) {$2$};
\node at (2.5,1.4) {$3$};
\node at (3.5,1.4) {$4$};
\node at (4.5,1.4) {$5$};
\node at (5.5,1.4) {$6$};
\node at (6.5,1.4) {$7$};
\node at (7.5,1.4) {$8$};
\end{tikzpicture}
\end{center}
The following code constructs a sum array
\texttt{s} for an array \texttt{t} in $O(n)$ time:
\begin{lstlisting}
for (int i = 1; i <= n; i++) {
    s[i] = s[i-1]+t[i];
}
\end{lstlisting}
After this, the following function processes
any sum query in $O(1)$ time:
\begin{lstlisting}
int sum(int a, int b) {
    return s[b]-s[a-1];
}
\end{lstlisting}

The function calculates the sum in the range $[a,b]$
by subtracting the sum in the range $[1,a-1]$
from the sum in the range $[1,b]$.
Thus, only two values of the sum array
are needed, and the query takes $O(1)$ time.
Note that because of the one-based indexing,
the function also works when $a=1$ if $\texttt{s}[0]=0$. 

As an example, consider the range $[4,7]$:
\begin{center}
\begin{tikzpicture}[scale=0.7]
\fill[color=lightgray] (3,0) rectangle (7,1);
\draw (0,0) grid (8,1);

\node at (0.5,0.5) {$1$};
\node at (1.5,0.5) {$3$};
\node at (2.5,0.5) {$4$};
\node at (3.5,0.5) {$8$};
\node at (4.5,0.5) {$6$};
\node at (5.5,0.5) {$1$};
\node at (6.5,0.5) {$4$};
\node at (7.5,0.5) {$2$};

\footnotesize
\node at (0.5,1.4) {$1$};
\node at (1.5,1.4) {$2$};
\node at (2.5,1.4) {$3$};
\node at (3.5,1.4) {$4$};
\node at (4.5,1.4) {$5$};
\node at (5.5,1.4) {$6$};
\node at (6.5,1.4) {$7$};
\node at (7.5,1.4) {$8$};
\end{tikzpicture}
\end{center}
The sum in the range is $8+6+1+4=19$.
This can be calculated using the precalculated
sums for the ranges $[1,3]$ and $[1,7]$:
\begin{center}
\begin{tikzpicture}[scale=0.7]
\fill[color=lightgray] (2,0) rectangle (3,1);
\fill[color=lightgray] (6,0) rectangle (7,1);
\draw (0,0) grid (8,1);

\node at (0.5,0.5) {$1$};
\node at (1.5,0.5) {$4$};
\node at (2.5,0.5) {$8$};
\node at (3.5,0.5) {$16$};
\node at (4.5,0.5) {$22$};
\node at (5.5,0.5) {$23$};
\node at (6.5,0.5) {$27$};
\node at (7.5,0.5) {$29$};


\footnotesize
\node at (0.5,1.4) {$1$};
\node at (1.5,1.4) {$2$};
\node at (2.5,1.4) {$3$};
\node at (3.5,1.4) {$4$};
\node at (4.5,1.4) {$5$};
\node at (5.5,1.4) {$6$};
\node at (6.5,1.4) {$7$};
\node at (7.5,1.4) {$8$};
\end{tikzpicture}
\end{center}
Thus, the sum in the range $[4,7]$ is $27-8=19$.

It is also possible to generalize this idea
to higher dimensions.
For example, we can construct a two-dimensional
sum array that can be used for calculating
the sum of any rectangular subarray in $O(1)$ time.
Each value in such an array is the sum of a subarray
that begins at the upper-left corner of the array.

\begin{samepage}
The following picture illustrates the idea:
\begin{center}
\begin{tikzpicture}[scale=0.54]
\draw[fill=lightgray] (3,2) rectangle (7,5);
\draw (0,0) grid (10,7);
%\draw[line width=2pt] (3,2) rectangle (7,5);
\node[anchor=center] at (6.5, 2.5) {$A$};
\node[anchor=center] at (2.5, 2.5) {$B$};
\node[anchor=center] at (6.5, 5.5) {$C$};
\node[anchor=center] at (2.5, 5.5) {$D$};
\end{tikzpicture}
\end{center}
\end{samepage}

The sum of the gray subarray can be calculated
using the formula
\[S(A) - S(B) - S(C) + S(D),\]
where $S(X)$ denotes the sum of a rectangular
subarray from the upper-left corner
to the position of $X$.

\subsubsection{Minimum query}

It is also possible to process minimum queries
in $O(1)$ time after preprocessing, though it is
more difficult than processing sum queries.
Note that minimum and maximum queries can always
be implemented using same techniques,
so it suffices to focus on minimum queries.

The idea is to precalculate the minimum element of each range
of size $2^k$ in the array.
For example, in the array

\begin{center}
\begin{tikzpicture}[scale=0.7]
\draw (0,0) grid (8,1);

\node at (0.5,0.5) {$1$};
\node at (1.5,0.5) {$3$};
\node at (2.5,0.5) {$4$};
\node at (3.5,0.5) {$8$};
\node at (4.5,0.5) {$6$};
\node at (5.5,0.5) {$1$};
\node at (6.5,0.5) {$4$};
\node at (7.5,0.5) {$2$};

\footnotesize
\node at (0.5,1.4) {$1$};
\node at (1.5,1.4) {$2$};
\node at (2.5,1.4) {$3$};
\node at (3.5,1.4) {$4$};
\node at (4.5,1.4) {$5$};
\node at (5.5,1.4) {$6$};
\node at (6.5,1.4) {$7$};
\node at (7.5,1.4) {$8$};
\end{tikzpicture}
\end{center}
the following minima will be calculated:

\begin{center}
\begin{tabular}{ccc}

\begin{tabular}{ccc}
range & size & min \\
\hline
$[1,1]$ & 1 & 1 \\
$[2,2]$ & 1 & 3 \\
$[3,3]$ & 1 & 4 \\
$[4,4]$ & 1 & 8 \\
$[5,5]$ & 1 & 6 \\
$[6,6]$ & 1 & 1 \\
$[7,7]$ & 1 & 4 \\
$[8,8]$ & 1 & 2 \\
\end{tabular}

&

\begin{tabular}{ccc}
range & size & min \\
\hline
$[1,2]$ & 2 & 1 \\
$[2,3]$ & 2 & 3 \\
$[3,4]$ & 2 & 4 \\
$[4,5]$ & 2 & 6 \\
$[5,6]$ & 2 & 1 \\
$[6,7]$ & 2 & 1 \\
$[7,8]$ & 2 & 2 \\
\\
\end{tabular}

&

\begin{tabular}{ccc}
range & size & min \\
\hline
$[1,4]$ & 4 & 1 \\
$[2,5]$ & 4 & 3 \\
$[3,6]$ & 4 & 1 \\
$[4,7]$ & 4 & 1 \\
$[5,8]$ & 4 & 1 \\
$[1,8]$ & 8 & 1 \\
\\
\\
\end{tabular}

\end{tabular}

\end{center}

There are $O(n \log n)$ ranges of size $2^k$,
because for each array position,
there are $O(\log n)$ ranges that begin at that position.
The minima in all ranges of size $2^k$ can be calculated
in $O(n \log n)$ time, because each range of size $2^k$
consists of two ranges of size $2^{k-1}$ and the minima
can be calculated recursively.

After this, the minimum in any range $[a,b]$
can be calculated in $O(1)$ time as a minimum of
two ranges of size $2^k$ where $k=\lfloor \log_2(b-a+1) \rfloor$.
The first range begins at index $a$,
and the second range ends at index $b$.
The parameter $k$ is chosen so that
the two ranges of size $2^k$
fully cover the range $[a,b]$.

As an example, consider the range $[2,7]$:
\begin{center}
\begin{tikzpicture}[scale=0.7]
\fill[color=lightgray] (1,0) rectangle (7,1);
\draw (0,0) grid (8,1);

\node at (0.5,0.5) {$1$};
\node at (1.5,0.5) {$3$};
\node at (2.5,0.5) {$4$};
\node at (3.5,0.5) {$8$};
\node at (4.5,0.5) {$6$};
\node at (5.5,0.5) {$1$};
\node at (6.5,0.5) {$4$};
\node at (7.5,0.5) {$2$};

\footnotesize
\node at (0.5,1.4) {$1$};
\node at (1.5,1.4) {$2$};
\node at (2.5,1.4) {$3$};
\node at (3.5,1.4) {$4$};
\node at (4.5,1.4) {$5$};
\node at (5.5,1.4) {$6$};
\node at (6.5,1.4) {$7$};
\node at (7.5,1.4) {$8$};
\end{tikzpicture}
\end{center}
The length of the range is 6,
and $\lfloor \log_2(6) \rfloor = 2$.
Thus, the minimum can be calculated
from two ranges of length 4.
The ranges are $[2,5]$ and $[4,7]$:
\begin{center}
\begin{tikzpicture}[scale=0.7]
\fill[color=lightgray] (1,0) rectangle (5,1);
\draw (0,0) grid (8,1);

\node at (0.5,0.5) {$1$};
\node at (1.5,0.5) {$3$};
\node at (2.5,0.5) {$4$};
\node at (3.5,0.5) {$8$};
\node at (4.5,0.5) {$6$};
\node at (5.5,0.5) {$1$};
\node at (6.5,0.5) {$4$};
\node at (7.5,0.5) {$2$};

\footnotesize
\node at (0.5,1.4) {$1$};
\node at (1.5,1.4) {$2$};
\node at (2.5,1.4) {$3$};
\node at (3.5,1.4) {$4$};
\node at (4.5,1.4) {$5$};
\node at (5.5,1.4) {$6$};
\node at (6.5,1.4) {$7$};
\node at (7.5,1.4) {$8$};
\end{tikzpicture}
\end{center}
\begin{center}
\begin{tikzpicture}[scale=0.7]
\fill[color=lightgray] (3,0) rectangle (7,1);
\draw (0,0) grid (8,1);

\node at (0.5,0.5) {$1$};
\node at (1.5,0.5) {$3$};
\node at (2.5,0.5) {$4$};
\node at (3.5,0.5) {$8$};
\node at (4.5,0.5) {$6$};
\node at (5.5,0.5) {$1$};
\node at (6.5,0.5) {$4$};
\node at (7.5,0.5) {$2$};


\footnotesize
\node at (0.5,1.4) {$1$};
\node at (1.5,1.4) {$2$};
\node at (2.5,1.4) {$3$};
\node at (3.5,1.4) {$4$};
\node at (4.5,1.4) {$5$};
\node at (5.5,1.4) {$6$};
\node at (6.5,1.4) {$7$};
\node at (7.5,1.4) {$8$};
\end{tikzpicture}
\end{center}
Since the minimum in the range $[2,5]$ is 3
and the minimum in the range $[4,7]$ is 1,
we know that the minimum in the range $[2,7]$ is 1.

\section{Binary indexed tree}

\index{binary indexed tree}
\index{Fenwick tree}

A \key{binary indexed tree} or \key{Fenwick tree}
can be seen as a dynamic version of a sum array.
The tree supports two $O(\log n)$ time operations:
calculating the sum of elements in a range,
and modifying the value of an element.

The benefit in using a binary indexed tree is
that the elements of the underlying array
can be efficiently updated between the queries.
This would not be possible with a sum array,
because after each update, we should build the
whole sum array again in $O(n)$ time.

\subsubsection{Structure}

Given an array of $n$ elements, indexed $1 \ldots n$,
the binary indexed tree for that array
is an array such that the value at position $k$
equals the sum of elements in the original array in a range
that ends at position $k$.
The length of the range is the largest power of two
that divides $k$.
For example, if $k=6$, the length of the range is $2$,
because $2$ divides $6$ but $4$ does not divide $6$.

\begin{samepage}
For example, consider the following array:
\begin{center}
\begin{tikzpicture}[scale=0.7]
\draw (0,0) grid (8,1);

\node at (0.5,0.5) {$1$};
\node at (1.5,0.5) {$3$};
\node at (2.5,0.5) {$4$};
\node at (3.5,0.5) {$8$};
\node at (4.5,0.5) {$6$};
\node at (5.5,0.5) {$1$};
\node at (6.5,0.5) {$4$};
\node at (7.5,0.5) {$2$};

\footnotesize
\node at (0.5,1.4) {$1$};
\node at (1.5,1.4) {$2$};
\node at (2.5,1.4) {$3$};
\node at (3.5,1.4) {$4$};
\node at (4.5,1.4) {$5$};
\node at (5.5,1.4) {$6$};
\node at (6.5,1.4) {$7$};
\node at (7.5,1.4) {$8$};
\end{tikzpicture}
\end{center}
\end{samepage}
The corresponding binary indexed tree is as follows:
\begin{center}
\begin{tikzpicture}[scale=0.7]
%\fill[color=lightgray] (3,0) rectangle (7,1);
\draw (0,0) grid (8,1);

\node at (0.5,0.5) {$1$};
\node at (1.5,0.5) {$4$};
\node at (2.5,0.5) {$4$};
\node at (3.5,0.5) {$16$};
\node at (4.5,0.5) {$6$};
\node at (5.5,0.5) {$7$};
\node at (6.5,0.5) {$4$};
\node at (7.5,0.5) {$29$};

\footnotesize
\node at (0.5,1.4) {$1$};
\node at (1.5,1.4) {$2$};
\node at (2.5,1.4) {$3$};
\node at (3.5,1.4) {$4$};
\node at (4.5,1.4) {$5$};
\node at (5.5,1.4) {$6$};
\node at (6.5,1.4) {$7$};
\node at (7.5,1.4) {$8$};

\draw[->,thick] (0.5,-0.9) -- (0.5,-0.1);
\draw[->,thick] (2.5,-0.9) -- (2.5,-0.1);
\draw[->,thick] (4.5,-0.9) -- (4.5,-0.1);
\draw[->,thick] (6.5,-0.9) -- (6.5,-0.1);
\draw[->,thick] (1.5,-1.9) -- (1.5,-0.1);
\draw[->,thick] (5.5,-1.9) -- (5.5,-0.1);
\draw[->,thick] (3.5,-2.9) -- (3.5,-0.1);
\draw[->,thick] (7.5,-3.9) -- (7.5,-0.1);

\draw (0,-1) -- (1,-1) -- (1,-1.5) -- (0,-1.5) -- (0,-1);
\draw (2,-1) -- (3,-1) -- (3,-1.5) -- (2,-1.5) -- (2,-1);
\draw (4,-1) -- (5,-1) -- (5,-1.5) -- (4,-1.5) -- (4,-1);
\draw (6,-1) -- (7,-1) -- (7,-1.5) -- (6,-1.5) -- (6,-1);
\draw (0,-2) -- (2,-2) -- (2,-2.5) -- (0,-2.5) -- (0,-2);
\draw (4,-2) -- (6,-2) -- (6,-2.5) -- (4,-2.5) -- (4,-2);
\draw (0,-3) -- (4,-3) -- (4,-3.5) -- (0,-3.5) -- (0,-3);
\draw (0,-4) -- (8,-4) -- (8,-4.5) -- (0,-4.5) -- (0,-4);
\end{tikzpicture}
\end{center}

For example, the value at position 6
in the binary indexed tree is 7,
because the sum of elements in the range $[5,6]$
in the original array is $6+1=7$.

\subsubsection{Sum query}

The basic operation in a binary indexed tree is
to calculate the sum of elements in a range $[1,k]$,
where $k$ is any position in the array.
The sum of such a range can be calculated as a
sum of one or more values stored in the tree.

For example, the range $[1,7]$ corresponds to
the following values:
\begin{center}
\begin{tikzpicture}[scale=0.7]
%\fill[color=lightgray] (3,0) rectangle (7,1);
\draw (0,0) grid (8,1);

\node at (0.5,0.5) {$1$};
\node at (1.5,0.5) {$4$};
\node at (2.5,0.5) {$4$};
\node at (3.5,0.5) {$16$};
\node at (4.5,0.5) {$6$};
\node at (5.5,0.5) {$7$};
\node at (6.5,0.5) {$4$};
\node at (7.5,0.5) {$29$};

\footnotesize
\node at (0.5,1.4) {$1$};
\node at (1.5,1.4) {$2$};
\node at (2.5,1.4) {$3$};
\node at (3.5,1.4) {$4$};
\node at (4.5,1.4) {$5$};
\node at (5.5,1.4) {$6$};
\node at (6.5,1.4) {$7$};
\node at (7.5,1.4) {$8$};

\draw[->,thick] (0.5,-0.9) -- (0.5,-0.1);
\draw[->,thick] (2.5,-0.9) -- (2.5,-0.1);
\draw[->,thick] (4.5,-0.9) -- (4.5,-0.1);
\draw[->,thick] (6.5,-0.9) -- (6.5,-0.1);
\draw[->,thick] (1.5,-1.9) -- (1.5,-0.1);
\draw[->,thick] (5.5,-1.9) -- (5.5,-0.1);
\draw[->,thick] (3.5,-2.9) -- (3.5,-0.1);
\draw[->,thick] (7.5,-3.9) -- (7.5,-0.1);

\draw (0,-1) -- (1,-1) -- (1,-1.5) -- (0,-1.5) -- (0,-1);
\draw (2,-1) -- (3,-1) -- (3,-1.5) -- (2,-1.5) -- (2,-1);
\draw (4,-1) -- (5,-1) -- (5,-1.5) -- (4,-1.5) -- (4,-1);
\draw[fill=lightgray] (6,-1) -- (7,-1) -- (7,-1.5) -- (6,-1.5) -- (6,-1);
\draw (0,-2) -- (2,-2) -- (2,-2.5) -- (0,-2.5) -- (0,-2);
\draw[fill=lightgray] (4,-2) -- (6,-2) -- (6,-2.5) -- (4,-2.5) -- (4,-2);
\draw[fill=lightgray] (0,-3) -- (4,-3) -- (4,-3.5) -- (0,-3.5) -- (0,-3);
\draw (0,-4) -- (8,-4) -- (8,-4.5) -- (0,-4.5) -- (0,-4);
\end{tikzpicture}
\end{center}

Hence, the sum of elements in the range $[1,7]$ is $16+7+4=27$.
The structure of the binary indexed tree allows us to calculate
the sum of elements in any range using only $O(\log n)$
values from the tree.

Using the same technique that we previously used
with a sum array,
we can efficiently calculate the sum of any range
$[a,b]$ by substracting the sum of the range $[1,a-1]$
from the sum of the range $[1,b]$.
Also here, only $O(\log n)$ values are needed,
because it suffices to calculate two sums of $[1,k]$ ranges.

\subsubsection{Array update}

When an element in the original array changes,
several sums in the binary indexed tree change.
For example, if the element at position 3 changes,
the sums of the following ranges change:
\begin{center}
\begin{tikzpicture}[scale=0.7]
%\fill[color=lightgray] (3,0) rectangle (7,1);
\draw (0,0) grid (8,1);

\node at (0.5,0.5) {$1$};
\node at (1.5,0.5) {$4$};
\node at (2.5,0.5) {$4$};
\node at (3.5,0.5) {$16$};
\node at (4.5,0.5) {$6$};
\node at (5.5,0.5) {$7$};
\node at (6.5,0.5) {$4$};
\node at (7.5,0.5) {$29$};

\footnotesize
\node at (0.5,1.4) {$1$};
\node at (1.5,1.4) {$2$};
\node at (2.5,1.4) {$3$};
\node at (3.5,1.4) {$4$};
\node at (4.5,1.4) {$5$};
\node at (5.5,1.4) {$6$};
\node at (6.5,1.4) {$7$};
\node at (7.5,1.4) {$8$};

\draw[->,thick] (0.5,-0.9) -- (0.5,-0.1);
\draw[->,thick] (2.5,-0.9) -- (2.5,-0.1);
\draw[->,thick] (4.5,-0.9) -- (4.5,-0.1);
\draw[->,thick] (6.5,-0.9) -- (6.5,-0.1);
\draw[->,thick] (1.5,-1.9) -- (1.5,-0.1);
\draw[->,thick] (5.5,-1.9) -- (5.5,-0.1);
\draw[->,thick] (3.5,-2.9) -- (3.5,-0.1);
\draw[->,thick] (7.5,-3.9) -- (7.5,-0.1);

\draw (0,-1) -- (1,-1) -- (1,-1.5) -- (0,-1.5) -- (0,-1);
\draw[fill=lightgray] (2,-1) -- (3,-1) -- (3,-1.5) -- (2,-1.5) -- (2,-1);
\draw (4,-1) -- (5,-1) -- (5,-1.5) -- (4,-1.5) -- (4,-1);
\draw (6,-1) -- (7,-1) -- (7,-1.5) -- (6,-1.5) -- (6,-1);
\draw (0,-2) -- (2,-2) -- (2,-2.5) -- (0,-2.5) -- (0,-2);
\draw (4,-2) -- (6,-2) -- (6,-2.5) -- (4,-2.5) -- (4,-2);
\draw[fill=lightgray] (0,-3) -- (4,-3) -- (4,-3.5) -- (0,-3.5) -- (0,-3);
\draw[fill=lightgray] (0,-4) -- (8,-4) -- (8,-4.5) -- (0,-4.5) -- (0,-4);
\end{tikzpicture}
\end{center}

However, it turns out that
the number of values that need to be updated
in the binary indexed tree is only $O(\log n)$.

\subsubsection{Implementation}

The operations of a binary indexed tree can be implemented
in an elegant and efficient way using bit operations.
The key fact needed is that $k \& -k$
isolates the last one bit in a number $k$.
For example, $6 \& -6=2$ because the number $6$
corresponds to 110 and the number $2$ corresponds to 10.

It turns out that when processing a range query,
the position $k$ in the binary indexed tree should be
decreased by $k \& -k$ at every step,
and when updating the array,
the position $k$ should be increased by $k \& -k$ at every step.

Suppose that the binary indexed tree is stored in an array \texttt{b}.
The following function calculates
the sum of elements in a range $[1,k]$:
\begin{lstlisting}
int sum(int k) {
    int s = 0;
    while (k >= 1) {
        s += b[k];
        k -= k&-k;
    }
    return s;
}
\end{lstlisting}

The following function increases the value
of the element at position $k$ by $x$
($x$ can be positive or negative):
\begin{lstlisting}
void add(int k, int x) {
    while (k <= n) {
        b[k] += x;
        k += k&-k;
    }
}
\end{lstlisting}

The time complexity of both the functions is
$O(\log n)$, because the functions access $O(\log n)$
values in the binary indexed tree, and each transition
to the next position
takes $O(1)$ time using bit operations.

\section{Segment tree}

\index{segment tree}

A \key{segment tree} is a data structure
that supports two operations:
processing a range query and
modifying an element in the array.
Segment trees can support
sum queries, minimum and maximum queries and many other
queries so that both operations work in $O(\log n)$ time.

Compared to a binary indexed tree,
the advantage of a segment tree is that it is
a more general data structure.
While binary indexed trees only support
sum queries, segment trees also support other queries.
On the other hand, a segment tree requires more
memory and is a bit more difficult to implement.

\subsubsection{Structure}

A segment tree is a binary tree that
contains $2n-1$ nodes.
The nodes on the bottom level of the tree
correspond to the original array,
and the other nodes
contain information needed for processing range queries.

We will first discuss segment trees that support
sum queries. As an example, consider the following array:
\begin{center}
\begin{tikzpicture}[scale=0.7]
\draw (0,0) grid (8,1);

\node at (0.5,0.5) {$5$};
\node at (1.5,0.5) {$8$};
\node at (2.5,0.5) {$6$};
\node at (3.5,0.5) {$3$};
\node at (4.5,0.5) {$2$};
\node at (5.5,0.5) {$7$};
\node at (6.5,0.5) {$2$};
\node at (7.5,0.5) {$6$};

% \footnotesize
% \node at (0.5,1.4) {$1$};
% \node at (1.5,1.4) {$2$};
% \node at (2.5,1.4) {$3$};
% \node at (3.5,1.4) {$4$};
% \node at (4.5,1.4) {$5$};
% \node at (5.5,1.4) {$6$};
% \node at (6.5,1.4) {$7$};
% \node at (7.5,1.4) {$8$};
\end{tikzpicture}
\end{center}
The corresponding segment tree is as follows:
\begin{center}
\begin{tikzpicture}[scale=0.7]
\draw (0,0) grid (8,1);

\node[anchor=center] at (0.5, 0.5) {5};
\node[anchor=center] at (1.5, 0.5) {8};
\node[anchor=center] at (2.5, 0.5) {6};
\node[anchor=center] at (3.5, 0.5) {3};
\node[anchor=center] at (4.5, 0.5) {2};
\node[anchor=center] at (5.5, 0.5) {7};
\node[anchor=center] at (6.5, 0.5) {2};
\node[anchor=center] at (7.5, 0.5) {6};

\node[draw, circle] (a) at (1,2.5) {13};
\path[draw,thick,-] (a) -- (0.5,1);
\path[draw,thick,-] (a) -- (1.5,1);
\node[draw, circle,minimum size=22pt] (b) at (3,2.5) {9};
\path[draw,thick,-] (b) -- (2.5,1);
\path[draw,thick,-] (b) -- (3.5,1);
\node[draw, circle,minimum size=22pt] (c) at (5,2.5) {9};
\path[draw,thick,-] (c) -- (4.5,1);
\path[draw,thick,-] (c) -- (5.5,1);
\node[draw, circle,minimum size=22pt] (d) at (7,2.5) {8};
\path[draw,thick,-] (d) -- (6.5,1);
\path[draw,thick,-] (d) -- (7.5,1);

\node[draw, circle] (i) at (2,4.5) {22};
\path[draw,thick,-] (i) -- (a);
\path[draw,thick,-] (i) -- (b);
\node[draw, circle] (j) at (6,4.5) {17};
\path[draw,thick,-] (j) -- (c);
\path[draw,thick,-] (j) -- (d);

\node[draw, circle] (m) at (4,6.5) {39};
\path[draw,thick,-] (m) -- (i);
\path[draw,thick,-] (m) -- (j);
\end{tikzpicture}
\end{center}

Each internal node in the segment tree contains
information about a range of size $2^k$
in the original array.
In the above tree, the value of each internal
node is the sum of the corresponding array elements,
and it can be calculated as the sum of
the values of its left and right child node.

It is convenient to build a segment tree
for an array whose size is a power of two,
because in this case every internal node has a left
and right child.
In the sequel, we will assume that the tree
is built like this.
If the size of the array is not a power of two,
we can always add zero elements to the array.

\subsubsection{Range query}

The sum of elements in a given range
can be calculated as a sum of values in the segment tree.
For example, consider the following range:
\begin{center}
\begin{tikzpicture}[scale=0.7]
\fill[color=gray!50] (2,0) rectangle (8,1);
\draw (0,0) grid (8,1);

\node[anchor=center] at (0.5, 0.5) {5};
\node[anchor=center] at (1.5, 0.5) {8};
\node[anchor=center] at (2.5, 0.5) {6};
\node[anchor=center] at (3.5, 0.5) {3};
\node[anchor=center] at (4.5, 0.5) {2};
\node[anchor=center] at (5.5, 0.5) {7};
\node[anchor=center] at (6.5, 0.5) {2};
\node[anchor=center] at (7.5, 0.5) {6};
% 
% \footnotesize
% \node at (0.5,1.4) {$1$};
% \node at (1.5,1.4) {$2$};
% \node at (2.5,1.4) {$3$};
% \node at (3.5,1.4) {$4$};
% \node at (4.5,1.4) {$5$};
% \node at (5.5,1.4) {$6$};
% \node at (6.5,1.4) {$7$};
% \node at (7.5,1.4) {$8$};
\end{tikzpicture}
\end{center}
The sum of elements in the range is
$6+3+2+7+2+6=26$.
The following two nodes in the tree
cover the range:
\begin{center}
\begin{tikzpicture}[scale=0.7]
\draw (0,0) grid (8,1);

\node[anchor=center] at (0.5, 0.5) {5};
\node[anchor=center] at (1.5, 0.5) {8};
\node[anchor=center] at (2.5, 0.5) {6};
\node[anchor=center] at (3.5, 0.5) {3};
\node[anchor=center] at (4.5, 0.5) {2};
\node[anchor=center] at (5.5, 0.5) {7};
\node[anchor=center] at (6.5, 0.5) {2};
\node[anchor=center] at (7.5, 0.5) {6};

\node[draw, circle] (a) at (1,2.5) {13};
\path[draw,thick,-] (a) -- (0.5,1);
\path[draw,thick,-] (a) -- (1.5,1);
\node[draw, circle,fill=gray!50,minimum size=22pt] (b) at (3,2.5) {9};
\path[draw,thick,-] (b) -- (2.5,1);
\path[draw,thick,-] (b) -- (3.5,1);
\node[draw, circle,minimum size=22pt] (c) at (5,2.5) {9};
\path[draw,thick,-] (c) -- (4.5,1);
\path[draw,thick,-] (c) -- (5.5,1);
\node[draw, circle,minimum size=22pt] (d) at (7,2.5) {8};
\path[draw,thick,-] (d) -- (6.5,1);
\path[draw,thick,-] (d) -- (7.5,1);

\node[draw, circle] (i) at (2,4.5) {22};
\path[draw,thick,-] (i) -- (a);
\path[draw,thick,-] (i) -- (b);
\node[draw, circle,fill=gray!50] (j) at (6,4.5) {17};
\path[draw,thick,-] (j) -- (c);
\path[draw,thick,-] (j) -- (d);

\node[draw, circle] (m) at (4,6.5) {39};
\path[draw,thick,-] (m) -- (i);
\path[draw,thick,-] (m) -- (j);
\end{tikzpicture}
\end{center}
Thus, the sum of elements in the range is $9+17=26$.

When the sum is calculated using nodes
that are located as high as possible in the tree,
at most two nodes on each level
of the tree are needed.
Hence, the total number of nodes
examined is only $O(\log n)$.

\subsubsection{Array update}

When an element in the array changes,
we should update all nodes in the tree
whose value depends on the element.
This can be done by traversing the path
from the element to the top node
and updating the nodes along the path.

\begin{samepage}
The following picture shows which nodes in the segment tree
change if the element 7 in the array changes.

\begin{center}
\begin{tikzpicture}[scale=0.7]
\fill[color=gray!50] (5,0) rectangle (6,1);
\draw (0,0) grid (8,1);

\node[anchor=center] at (0.5, 0.5) {5};
\node[anchor=center] at (1.5, 0.5) {8};
\node[anchor=center] at (2.5, 0.5) {6};
\node[anchor=center] at (3.5, 0.5) {3};
\node[anchor=center] at (4.5, 0.5) {2};
\node[anchor=center] at (5.5, 0.5) {7};
\node[anchor=center] at (6.5, 0.5) {2};
\node[anchor=center] at (7.5, 0.5) {6};

\node[draw, circle] (a) at (1,2.5) {13};
\path[draw,thick,-] (a) -- (0.5,1);
\path[draw,thick,-] (a) -- (1.5,1);
\node[draw, circle,minimum size=22pt] (b) at (3,2.5) {9};
\path[draw,thick,-] (b) -- (2.5,1);
\path[draw,thick,-] (b) -- (3.5,1);
\node[draw, circle,minimum size=22pt,fill=gray!50] (c) at (5,2.5) {9};
\path[draw,thick,-] (c) -- (4.5,1);
\path[draw,thick,-] (c) -- (5.5,1);
\node[draw, circle,minimum size=22pt] (d) at (7,2.5) {8};
\path[draw,thick,-] (d) -- (6.5,1);
\path[draw,thick,-] (d) -- (7.5,1);

\node[draw, circle] (i) at (2,4.5) {22};
\path[draw,thick,-] (i) -- (a);
\path[draw,thick,-] (i) -- (b);
\node[draw, circle,fill=gray!50] (j) at (6,4.5) {17};
\path[draw,thick,-] (j) -- (c);
\path[draw,thick,-] (j) -- (d);

\node[draw, circle,fill=gray!50] (m) at (4,6.5) {39};
\path[draw,thick,-] (m) -- (i);
\path[draw,thick,-] (m) -- (j);
\end{tikzpicture}
\end{center}
\end{samepage}

The path from bottom to top
always consists of $O(\log n)$ nodes,
so each update changes $O(\log n)$ nodes in the tree.

\subsubsection{Storing the tree}

A segment tree can be stored in an array
of $2N$ elements where $N$ is a power of two.
From now on, we will assume that the indices
of the original array are between $0$ and $N-1$.

The element at position 1 in the array
corresponds to the top node of the tree,
the elements at positions 2 and 3 correspond to
the second level of the tree, and so on.
Finally, the elements at positions $N \ldots 2N-1$
correspond to the bottom level of the tree, i.e.,
the elements of the original array.

For example, the segment tree
\begin{center}
\begin{tikzpicture}[scale=0.7]
\draw (0,0) grid (8,1);

\node[anchor=center] at (0.5, 0.5) {5};
\node[anchor=center] at (1.5, 0.5) {8};
\node[anchor=center] at (2.5, 0.5) {6};
\node[anchor=center] at (3.5, 0.5) {3};
\node[anchor=center] at (4.5, 0.5) {2};
\node[anchor=center] at (5.5, 0.5) {7};
\node[anchor=center] at (6.5, 0.5) {2};
\node[anchor=center] at (7.5, 0.5) {6};

\node[draw, circle] (a) at (1,2.5) {13};
\path[draw,thick,-] (a) -- (0.5,1);
\path[draw,thick,-] (a) -- (1.5,1);
\node[draw, circle,minimum size=22pt] (b) at (3,2.5) {9};
\path[draw,thick,-] (b) -- (2.5,1);
\path[draw,thick,-] (b) -- (3.5,1);
\node[draw, circle,minimum size=22pt] (c) at (5,2.5) {9};
\path[draw,thick,-] (c) -- (4.5,1);
\path[draw,thick,-] (c) -- (5.5,1);
\node[draw, circle,minimum size=22pt] (d) at (7,2.5) {8};
\path[draw,thick,-] (d) -- (6.5,1);
\path[draw,thick,-] (d) -- (7.5,1);

\node[draw, circle] (i) at (2,4.5) {22};
\path[draw,thick,-] (i) -- (a);
\path[draw,thick,-] (i) -- (b);
\node[draw, circle] (j) at (6,4.5) {17};
\path[draw,thick,-] (j) -- (c);
\path[draw,thick,-] (j) -- (d);

\node[draw, circle] (m) at (4,6.5) {39};
\path[draw,thick,-] (m) -- (i);
\path[draw,thick,-] (m) -- (j);
\end{tikzpicture}
\end{center}
can be stored as follows ($N=8$):
\begin{center}
\begin{tikzpicture}[scale=0.7]
%\fill[color=lightgray] (3,0) rectangle (7,1);
\draw (0,0) grid (15,1);

\node at (0.5,0.5) {$39$};
\node at (1.5,0.5) {$22$};
\node at (2.5,0.5) {$17$};
\node at (3.5,0.5) {$13$};
\node at (4.5,0.5) {$9$};
\node at (5.5,0.5) {$9$};
\node at (6.5,0.5) {$8$};
\node at (7.5,0.5) {$5$};
\node at (8.5,0.5) {$8$};
\node at (9.5,0.5) {$6$};
\node at (10.5,0.5) {$3$};
\node at (11.5,0.5) {$2$};
\node at (12.5,0.5) {$7$};
\node at (13.5,0.5) {$2$};
\node at (14.5,0.5) {$6$};

\footnotesize
\node at (0.5,1.4) {$1$};
\node at (1.5,1.4) {$2$};
\node at (2.5,1.4) {$3$};
\node at (3.5,1.4) {$4$};
\node at (4.5,1.4) {$5$};
\node at (5.5,1.4) {$6$};
\node at (6.5,1.4) {$7$};
\node at (7.5,1.4) {$8$};
\node at (8.5,1.4) {$9$};
\node at (9.5,1.4) {$10$};
\node at (10.5,1.4) {$11$};
\node at (11.5,1.4) {$12$};
\node at (12.5,1.4) {$13$};
\node at (13.5,1.4) {$14$};
\node at (14.5,1.4) {$15$};
\end{tikzpicture}
\end{center}
Using this representation,
for a node at position $k$,
\begin{itemize}
\item the parent node is at position $\lfloor k/2 \rfloor$,
\item the left child node is at position $2k$, and
\item the right child node is at position $2k+1$.
\end{itemize}
Note that this implies that the index of a node
is even if it is a left child and odd if it is a right child.

\subsubsection{Functions}

We assume that the segment tree is stored
in an array \texttt{p}.
The following function
calculates the sum of elements in a range $[a,b]$:

\begin{lstlisting}
int sum(int a, int b) {
    a += N; b += N;
    int s = 0;
    while (a <= b) {
        if (a%2 == 1) s += p[a++];
        if (b%2 == 0) s += p[b--];
        a /= 2; b /= 2;
    }
    return s;
}
\end{lstlisting}

The function maintains a range in the segment tree array.
Initially the range is $[a+N,b+N]$,
that corresponds to the range $[a,b]$
in the underlying array.
At each step, the function adds the value of
the left and right node to the sum
if their parent nodes do not belong to the range.
After this, the same process continues on the
next level of the tree.

The following function increases the value
of the element at position $k$ by $x$:

\begin{lstlisting}
void add(int k, int x) {
    k += N;
    p[k] += x;
    for (k /= 2; k >= 1; k /= 2) {
        p[k] = p[2*k]+p[2*k+1];
    }
}
\end{lstlisting}
First the function updates the element
at the bottom level of the tree.
After this, the function updates the values of all
internal nodes in the tree, until it reaches
the top node of the tree.

Both above functions work
in $O(\log n)$ time, because a segment tree
of $n$ elements consists of $O(\log n)$ levels,
and the operations move one level forward in the tree at each step.

\subsubsection{Other queries}

A segment tree can support any query
where the answer for a range $[a,b]$
can be calculated
from the answers for ranges $[a,c]$ and $[c+1,b]$, where
$c$ is some index between $a$ and $b$.
Examples of such queries are
minimum and maximum, greatest common divisor,
and bit operations and, or and xor.

\begin{samepage}
For example, the following segment tree
supports minimum queries:

\begin{center}
\begin{tikzpicture}[scale=0.7]
\draw (0,0) grid (8,1);

\node[anchor=center] at (0.5, 0.5) {5};
\node[anchor=center] at (1.5, 0.5) {8};
\node[anchor=center] at (2.5, 0.5) {6};
\node[anchor=center] at (3.5, 0.5) {3};
\node[anchor=center] at (4.5, 0.5) {1};
\node[anchor=center] at (5.5, 0.5) {7};
\node[anchor=center] at (6.5, 0.5) {2};
\node[anchor=center] at (7.5, 0.5) {6};

\node[draw, circle,minimum size=22pt] (a) at (1,2.5) {5};
\path[draw,thick,-] (a) -- (0.5,1);
\path[draw,thick,-] (a) -- (1.5,1);
\node[draw, circle,minimum size=22pt] (b) at (3,2.5) {3};
\path[draw,thick,-] (b) -- (2.5,1);
\path[draw,thick,-] (b) -- (3.5,1);
\node[draw, circle,minimum size=22pt] (c) at (5,2.5) {1};
\path[draw,thick,-] (c) -- (4.5,1);
\path[draw,thick,-] (c) -- (5.5,1);
\node[draw, circle,minimum size=22pt] (d) at (7,2.5) {2};
\path[draw,thick,-] (d) -- (6.5,1);
\path[draw,thick,-] (d) -- (7.5,1);

\node[draw, circle,minimum size=22pt] (i) at (2,4.5) {3};
\path[draw,thick,-] (i) -- (a);
\path[draw,thick,-] (i) -- (b);
\node[draw, circle,minimum size=22pt] (j) at (6,4.5) {1};
\path[draw,thick,-] (j) -- (c);
\path[draw,thick,-] (j) -- (d);

\node[draw, circle,minimum size=22pt] (m) at (4,6.5) {1};
\path[draw,thick,-] (m) -- (i);
\path[draw,thick,-] (m) -- (j);
\end{tikzpicture}
\end{center}
\end{samepage}

In this segment tree, every node in the tree
contains the smallest element in the corresponding
range of the underlying array.
The top node of the tree contains the smallest
element in the whole array.
The operations can be implemented like previously,
but instead of sums, minima are calculated.

\subsubsection{Binary search in tree}

The structure of the segment tree allows us
to use binary search for finding elements in the array.
For example, if the tree supports the minimum query,
we can find the position of the smallest
element in $O(\log n)$ time.

For example, in the following tree the
smallest element 1 can be found
by traversing a path downwards from the top node:

\begin{center}
\begin{tikzpicture}[scale=0.7]
\draw (8,0) grid (16,1);

\node[anchor=center] at (8.5, 0.5) {9};
\node[anchor=center] at (9.5, 0.5) {5};
\node[anchor=center] at (10.5, 0.5) {7};
\node[anchor=center] at (11.5, 0.5) {1};
\node[anchor=center] at (12.5, 0.5) {6};
\node[anchor=center] at (13.5, 0.5) {2};
\node[anchor=center] at (14.5, 0.5) {3};
\node[anchor=center] at (15.5, 0.5) {2};

%\node[anchor=center] at (1,2.5) {13};

\node[draw, circle,minimum size=22pt] (e) at (9,2.5) {5};
\path[draw,thick,-] (e) -- (8.5,1);
\path[draw,thick,-] (e) -- (9.5,1);
\node[draw, circle,minimum size=22pt] (f) at (11,2.5) {1};
\path[draw,thick,-] (f) -- (10.5,1);
\path[draw,thick,-] (f) -- (11.5,1);
\node[draw, circle,minimum size=22pt] (g) at (13,2.5) {2};
\path[draw,thick,-] (g) -- (12.5,1);
\path[draw,thick,-] (g) -- (13.5,1);
\node[draw, circle,minimum size=22pt] (h) at (15,2.5) {2};
\path[draw,thick,-] (h) -- (14.5,1);
\path[draw,thick,-] (h) -- (15.5,1);

\node[draw, circle,minimum size=22pt] (k) at (10,4.5) {1};
\path[draw,thick,-] (k) -- (e);
\path[draw,thick,-] (k) -- (f);
\node[draw, circle,minimum size=22pt] (l) at (14,4.5) {2};
\path[draw,thick,-] (l) -- (g);
\path[draw,thick,-] (l) -- (h);

\node[draw, circle,minimum size=22pt] (n) at (12,6.5) {1};
\path[draw,thick,-] (n) -- (k);
\path[draw,thick,-] (n) -- (l);


\path[draw=red,thick,->,line width=2pt] (n) -- (k);
\path[draw=red,thick,->,line width=2pt] (k) -- (f);
\path[draw=red,thick,->,line width=2pt] (f) -- (11.5,1);
\end{tikzpicture}
\end{center}

\section{Additional techniques}

\subsubsection{Index compression}

A limitation in data structures that
are built upon an array is that
the elements are indexed using integers
$1,2,3,$ etc.
Difficulties arise when large indices
are needed.
For example, if we wish to use the index $10^9$,
the array should contain $10^9$
elements which is not realistic.

\index{index compression}

However, we can often bypass this limitation
by using \key{index compression},
where the original indices are replaced
with the indices $1,2,3,$ etc.
This can be done if we know all the indices
needed during the algorithm beforehand.

The idea is to replace each original index $x$
with $p(x)$ where $p$ is a function that
compresses the indices.
We require that the order of the indices
does not change, so if $a<b$, then $p(a)<p(b)$.
This allows us to conviently perform queries
despite the fact that the indices are compressed.

For example, if the original indices are
$555$, $10^9$ and $8$, the new indices are:

\[
\begin{array}{lcl}
p(8) & = & 1 \\
p(555) & = & 2 \\
p(10^9) & = & 3 \\
\end{array}
\]

\subsubsection{Range update}

So far, we have implemented data structures
that support range queries and modifications
of single values.
Let us now consider a reverse situation,
where we should update ranges and
retrieve single values.
We focus on an operation that increases all
elements in a range $[a,b]$ by $x$.

Surprisingly, we can use the data structures
presented in this chapter also in this situation.
To do this, we change the array so that
each element indicates the \emph{change}
with respect to the previous element.
For example, the array

\begin{center}
\begin{tikzpicture}[scale=0.7]
\draw (0,0) grid (8,1);

\node at (0.5,0.5) {$3$};
\node at (1.5,0.5) {$3$};
\node at (2.5,0.5) {$1$};
\node at (3.5,0.5) {$1$};
\node at (4.5,0.5) {$1$};
\node at (5.5,0.5) {$5$};
\node at (6.5,0.5) {$2$};
\node at (7.5,0.5) {$2$};


\footnotesize
\node at (0.5,1.4) {$1$};
\node at (1.5,1.4) {$2$};
\node at (2.5,1.4) {$3$};
\node at (3.5,1.4) {$4$};
\node at (4.5,1.4) {$5$};
\node at (5.5,1.4) {$6$};
\node at (6.5,1.4) {$7$};
\node at (7.5,1.4) {$8$};
\end{tikzpicture}
\end{center}
becomes as follows:
\begin{center}
\begin{tikzpicture}[scale=0.7]
\draw (0,0) grid (8,1);

\node at (0.5,0.5) {$3$};
\node at (1.5,0.5) {$0$};
\node at (2.5,0.5) {$-2$};
\node at (3.5,0.5) {$0$};
\node at (4.5,0.5) {$0$};
\node at (5.5,0.5) {$4$};
\node at (6.5,0.5) {$-3$};
\node at (7.5,0.5) {$0$};


\footnotesize
\node at (0.5,1.4) {$1$};
\node at (1.5,1.4) {$2$};
\node at (2.5,1.4) {$3$};
\node at (3.5,1.4) {$4$};
\node at (4.5,1.4) {$5$};
\node at (5.5,1.4) {$6$};
\node at (6.5,1.4) {$7$};
\node at (7.5,1.4) {$8$};
\end{tikzpicture}
\end{center}

The original array is the sum array of the new array.
Thus, each element in the original array equals
a sum of values in the new array.
For example, the value 5 at position 6 in the original array
corresponds to the sum $3-2+4=5$.

The benefit in using the new array is
that we can update a range by changing just
two elements in the array.
For example, if we want to 
increase the elements in the range $2 \ldots 5$ by 5,
it suffices to increase the value at position 2 by 5
and decrease the value at position 6 by 5.
The result is as follows:

\begin{center}
\begin{tikzpicture}[scale=0.7]
\draw (0,0) grid (8,1);

\node at (0.5,0.5) {$3$};
\node at (1.5,0.5) {$5$};
\node at (2.5,0.5) {$-2$};
\node at (3.5,0.5) {$0$};
\node at (4.5,0.5) {$0$};
\node at (5.5,0.5) {$-1$};
\node at (6.5,0.5) {$-3$};
\node at (7.5,0.5) {$0$};

\footnotesize
\node at (0.5,1.4) {$1$};
\node at (1.5,1.4) {$2$};
\node at (2.5,1.4) {$3$};
\node at (3.5,1.4) {$4$};
\node at (4.5,1.4) {$5$};
\node at (5.5,1.4) {$6$};
\node at (6.5,1.4) {$7$};
\node at (7.5,1.4) {$8$};
\end{tikzpicture}
\end{center}

More generally, to increase the elements
in a range $[a,b]$ by $x$,
we increase the value at position $a$ by $x$
and decrease the value at position $b+1$ by $x$.
Thus, it is only needed to update single values
and process sum queries,
so we can use a binary indexed tree or a segment tree.

A more difficult problem is to support both
range queries and range updates.
In Chapter 28 we will see that even this is possible.