cphb/luku11.tex

\chapter{Basics of graphs}

Many programming problems can be solved by
interpreting the problem as a graph problem
and using a suitable graph algorithm.
A typical example of a graph is a network
of roads and cities in a country.
Sometimes, though, the graph is hidden
in the problem and it can be difficult to detect it.

This part of the book discusses techniques and algorithms
involving graphs
that are important in competitive programming.
We will first go through graph terminology
and different ways to store graphs in algorithms.

\section{Terminology}

\index{graph}
\index{node}
\index{edge}

A \key{graph} consists of \key{nodes}
and \key{edges} between them.
In this book,
the variable $n$ denotes the number of nodes
in a graph, and the variable $m$ denotes
the number of edges.
In addition, the nodes are numbered
using integers $1,2,\ldots,n$.

For example, the following graph contains 5 nodes and 7 edges:

\begin{center}
\begin{tikzpicture}[scale=0.9]
\node[draw, circle] (1) at (1,3) {$1$};
\node[draw, circle] (2) at (4,3) {$2$};
\node[draw, circle] (3) at (1,1) {$3$};
\node[draw, circle] (4) at (4,1) {$4$};
\node[draw, circle] (5) at (6,2) {$5$};

\path[draw,thick,-] (1) -- (2);
\path[draw,thick,-] (1) -- (3);
\path[draw,thick,-] (1) -- (4);
\path[draw,thick,-] (3) -- (4);
\path[draw,thick,-] (2) -- (4);
\path[draw,thick,-] (2) -- (5);
\path[draw,thick,-] (4) -- (5);
\end{tikzpicture}
\end{center}

\index{path}

A \key{path} is a route from node $a$ to node $b$
that goes through the edges in the graph.
The \key{length} of a path is the number of
edges in the path.
For example, in the above graph, paths
from node 1 to node 5 are:

\begin{itemize}
\item $1 \rightarrow 2 \rightarrow 5$ (length 2)
\item $1 \rightarrow 4 \rightarrow 5$ (length 2)
\item $1 \rightarrow 2 \rightarrow 4 \rightarrow 5$ (length 3)
\item $1 \rightarrow 3 \rightarrow 4 \rightarrow 5$ (length 3)
\item $1 \rightarrow 4 \rightarrow 2 \rightarrow 5$ (length 3)
\item $1 \rightarrow 3 \rightarrow 4 \rightarrow 2 \rightarrow 5$ (length 4)
\end{itemize}

\subsubsection{Connectivity}

\index{connected graph}

A graph is \key{connected}, if there is path
between any two nodes.
For example, the following graph is connected:
\begin{center}
\begin{tikzpicture}[scale=0.9]
\node[draw, circle] (1) at (1,3) {$1$};
\node[draw, circle] (2) at (4,3) {$2$};
\node[draw, circle] (3) at (1,1) {$3$};
\node[draw, circle] (4) at (4,1) {$4$};
\path[draw,thick,-] (1) -- (2);
\path[draw,thick,-] (1) -- (3);
\path[draw,thick,-] (2) -- (3);
\path[draw,thick,-] (3) -- (4);
\path[draw,thick,-] (2) -- (4);
\end{tikzpicture}
\end{center}

The following graph is not connected
because it is not possible to get to other
nodes from node 4.
\begin{center}
\begin{tikzpicture}[scale=0.9]
\node[draw, circle] (1) at (1,3) {$1$};
\node[draw, circle] (2) at (4,3) {$2$};
\node[draw, circle] (3) at (1,1) {$3$};
\node[draw, circle] (4) at (4,1) {$4$};
\path[draw,thick,-] (1) -- (2);
\path[draw,thick,-] (1) -- (3);
\path[draw,thick,-] (2) -- (3);
\end{tikzpicture}
\end{center}

\index{compomnent}

The connected parts of a graph are
its \key{components}.
For example, the following graph
contains three components:
$\{1,\,2,\,3\}$,
$\{4,\,5,\,6,\,7\}$ and
$\{8\}$.
\begin{center}
\begin{tikzpicture}[scale=0.8]
\node[draw, circle] (1) at (1,3) {$1$};
\node[draw, circle] (2) at (4,3) {$2$};
\node[draw, circle] (3) at (1,1) {$3$};

\node[draw, circle] (6) at (6,1) {$6$};
\node[draw, circle] (7) at (9,1) {$7$};
\node[draw, circle] (4) at (6,3) {$4$};
\node[draw, circle] (5) at (9,3) {$5$};

\node[draw, circle] (8) at (11,2) {$8$};

\path[draw,thick,-] (1) -- (2);
\path[draw,thick,-] (2) -- (3);
\path[draw,thick,-] (1) -- (3);
\path[draw,thick,-] (4) -- (5);
\path[draw,thick,-] (5) -- (7);
\path[draw,thick,-] (6) -- (7);
\path[draw,thick,-] (6) -- (4);
\end{tikzpicture}
\end{center}

\index{tree}

A \key{tree} is a connected graph
that contains $n$ nodes and $n-1$ edges.
In a tree, there is a unique path
between any two nodes.
For example, the following graph is a tree:

\begin{center}
\begin{tikzpicture}[scale=0.9]
\node[draw, circle] (1) at (1,3) {$1$};
\node[draw, circle] (2) at (4,3) {$2$};
\node[draw, circle] (3) at (1,1) {$3$};
\node[draw, circle] (4) at (4,1) {$4$};
\node[draw, circle] (5) at (6,2) {$5$};

\path[draw,thick,-] (1) -- (2);
\path[draw,thick,-] (1) -- (3);
%\path[draw,thick,-] (1) -- (4);
\path[draw,thick,-] (2) -- (5);
\path[draw,thick,-] (2) -- (4);
%\path[draw,thick,-] (4) -- (5);
\end{tikzpicture}
\end{center}

\subsubsection{Edge directions}

\index{directed graph}

A graph is \key{directed}
if the edges can be travelled only
in one direction.
For example, the following graph is directed:
\begin{center}
\begin{tikzpicture}[scale=0.9]
\node[draw, circle] (1) at (1,3) {$1$};
\node[draw, circle] (2) at (4,3) {$2$};
\node[draw, circle] (3) at (1,1) {$3$};
\node[draw, circle] (4) at (4,1) {$4$};
\node[draw, circle] (5) at (6,2) {$5$};
\path[draw,thick,->,>=latex] (1) -- (2);
\path[draw,thick,->,>=latex] (2) -- (4);
\path[draw,thick,->,>=latex] (2) -- (5);
\path[draw,thick,->,>=latex] (4) -- (5);
\path[draw,thick,->,>=latex] (4) -- (1);
\path[draw,thick,->,>=latex] (3) -- (1);
\end{tikzpicture}
\end{center}

The above graph contains a path from
node $3$ to $5$ using edges
$3 \rightarrow 1 \rightarrow 2 \rightarrow 5$.
However, the graph doesn't contain
a path from node $5$ to $3$.

\index{cycle}
\index{acyclic graph}

A \key{cycle} is a path whose first and
last node is the same.
For example, the above graph contains
a cycle
$1 \rightarrow 2 \rightarrow 4 \rightarrow 1$.
If a graph doesn't contain any cycles,
it is called \key{acyclic}.

\subsubsection{Edge weights}

\index{weighted graph}

In a \key{weighted} graph, each edge is assigned
a \key{weight}.
Often, the weights are interpreted as edge lengths.
For example, the following graph is weighted:
\begin{center}
\begin{tikzpicture}[scale=0.9]
\node[draw, circle] (1) at (1,3) {$1$};
\node[draw, circle] (2) at (4,3) {$2$};
\node[draw, circle] (3) at (1,1) {$3$};
\node[draw, circle] (4) at (4,1) {$4$};
\node[draw, circle] (5) at (6,2) {$5$};
\path[draw,thick,-] (1) -- node[font=\small,label=above:5] {} (2);
\path[draw,thick,-] (1) -- node[font=\small,label=left:1] {} (3);
\path[draw,thick,-] (3) -- node[font=\small,label=below:7] {} (4);
\path[draw,thick,-] (2) -- node[font=\small,label=left:6] {} (4);
\path[draw,thick,-] (2) -- node[font=\small,label=above:7] {} (5);
\path[draw,thick,-] (4) -- node[font=\small,label=below:3] {} (5);
\end{tikzpicture}
\end{center}

Now the length of a path is the sum of
edge weights.
For example, in the above graph
the length of path
$1 \rightarrow 2 \rightarrow 5$
is $12$, and the length of path
$1 \rightarrow 3 \rightarrow 4 \rightarrow 5$ is $11$.
The latter is the shortest path from node $1$ to node $5$.

\subsubsection{Neighbors and degrees}

\index{neighbor}
\index{degree}

Two nodes are \key{neighbors} or \key{adjacent}
if there is a edge between them.
The \key{degree} of a node
is the number of its neighbors.
For example, in the following graph,
the neighbors of node 2 are 1, 4 and 5,
so its degree is 3.

\begin{center}
\begin{tikzpicture}[scale=0.9]
\node[draw, circle] (1) at (1,3) {$1$};
\node[draw, circle] (2) at (4,3) {$2$};
\node[draw, circle] (3) at (1,1) {$3$};
\node[draw, circle] (4) at (4,1) {$4$};
\node[draw, circle] (5) at (6,2) {$5$};

\path[draw,thick,-] (1) -- (2);
\path[draw,thick,-] (1) -- (3);
\path[draw,thick,-] (1) -- (4);
\path[draw,thick,-] (3) -- (4);
\path[draw,thick,-] (2) -- (4);
\path[draw,thick,-] (2) -- (5);
%\path[draw,thick,-] (4) -- (5);
\end{tikzpicture}
\end{center}

The sum of degrees in a graph is always $2m$
where $m$ is the number of edges.
The reason for this is that each edge
increases the degree of two nodes by one.
Thus, the sum of degrees is always even.

\index{regular graph}
\index{complete graph}

A graph is \key{regular} if the
degree of every node is a constant $d$.
A graph is \key{complete} if the
degree of every node is $n-1$, i.e.,
the graph contains all possible edges
between the nodes.

\index{indegree}
\index{outdegree}

In a directed graph, the \key{indegree}
and \key{outdegree} of a node is
the number of edges that end and begin
at the node, respectively.
For example, in the following graph,
node 2 has indegree 2 and outdegree 1.

\begin{center}
\begin{tikzpicture}[scale=0.9]
\node[draw, circle] (1) at (1,3) {$1$};
\node[draw, circle] (2) at (4,3) {$2$};
\node[draw, circle] (3) at (1,1) {$3$};
\node[draw, circle] (4) at (4,1) {$4$};
\node[draw, circle] (5) at (6,2) {$5$};

\path[draw,thick,->,>=latex] (1) -- (2);
\path[draw,thick,->,>=latex] (1) -- (3);
\path[draw,thick,->,>=latex] (1) -- (4);
\path[draw,thick,->,>=latex] (3) -- (4);
\path[draw,thick,->,>=latex] (2) -- (4);
\path[draw,thick,<-,>=latex] (2) -- (5);
\end{tikzpicture}
\end{center}

\subsubsection{Colorings}

\index{coloring}
\index{bipartite graph}

In a \key{coloring} of a graph,
each node is assigned a color so that
no adjacent nodes have the same color.

A graph is \key{bipartite} if
it is possible to color it using two colors.
It turns out that a graph is bipartite
exactly when it doesn't contain a cycle
with odd number of edges.
For example, the graph
\begin{center}
\begin{tikzpicture}[scale=0.9]
\node[draw, circle] (1) at (1,3) {$2$};
\node[draw, circle] (2) at (4,3) {$3$};
\node[draw, circle] (3) at (1,1) {$5$};
\node[draw, circle] (4) at (4,1) {$6$};
\node[draw, circle] (5) at (-2,1) {$4$};
\node[draw, circle] (6) at (-2,3) {$1$};
\path[draw,thick,-] (1) -- (2);
\path[draw,thick,-] (1) -- (3);
\path[draw,thick,-] (3) -- (4);
\path[draw,thick,-] (2) -- (4);
\path[draw,thick,-] (3) -- (6);
\path[draw,thick,-] (5) -- (6);
\end{tikzpicture}
\end{center}
is bipartite because we can color it as follows:
\begin{center}
\begin{tikzpicture}[scale=0.9]
\node[draw, circle, fill=blue!40] (1) at (1,3) {$2$};
\node[draw, circle, fill=red!40] (2) at (4,3) {$3$};
\node[draw, circle, fill=red!40] (3) at (1,1) {$5$};
\node[draw, circle, fill=blue!40] (4) at (4,1) {$6$};
\node[draw, circle, fill=red!40] (5) at (-2,1) {$4$};
\node[draw, circle, fill=blue!40] (6) at (-2,3) {$1$};
\path[draw,thick,-] (1) -- (2);
\path[draw,thick,-] (1) -- (3);
\path[draw,thick,-] (3) -- (4);
\path[draw,thick,-] (2) -- (4);
\path[draw,thick,-] (3) -- (6);
\path[draw,thick,-] (5) -- (6);
\end{tikzpicture}
\end{center}

\subsubsection{Simplicity}

\index{simple graph}

A graph is \key{simple}
if no edge begins and ends at the same node,
and there are no multiple
edges between two nodes.
Often we will assume that the graph is simple.
For example, the graph
\begin{center}
\begin{tikzpicture}[scale=0.9]
\node[draw, circle] (1) at (1,3) {$2$};
\node[draw, circle] (2) at (4,3) {$3$};
\node[draw, circle] (3) at (1,1) {$5$};
\node[draw, circle] (4) at (4,1) {$6$};
\node[draw, circle] (5) at (-2,1) {$4$};
\node[draw, circle] (6) at (-2,3) {$1$};

\path[draw,thick,-] (1) edge [bend right=20] (2);
\path[draw,thick,-] (2) edge [bend right=20] (1);
%\path[draw,thick,-] (1) -- (2);
\path[draw,thick,-] (1) -- (3);
\path[draw,thick,-] (3) -- (4);
\path[draw,thick,-] (2) -- (4);
\path[draw,thick,-] (3) -- (6);
\path[draw,thick,-] (5) -- (6);

\tikzset{every loop/.style={in=135,out=190}}
\path[draw,thick,-] (5) edge [loop left] (5);
\end{tikzpicture}
\end{center}
is \emph{not} simple because there is an edge that begins
and ends at node 4, and there are two edges
between nodes 2 and 3.

\section{Verkko muistissa}

On monia tapoja pitää verkkoa muistissa algoritmissa.
Sopiva tietorakenne riippuu siitä,
kuinka suuri verkko on ja
millä tavoin algoritmi käsittelee sitä.
Seuraavaksi käymme läpi kolme tavallista vaihtoehtoa.

\subsubsection{Vieruslistaesitys}

\index{vieruslista@vieruslista}

Tavallisin tapa pitää verkkoa muistissa on
luoda jokaisesta solmusta \key{vieruslista},
joka sisältää kaikki solmut,
joihin solmusta pystyy siirtymään kaarta pitkin.
Vieruslistaesitys on tavallisin verkon esitysmuoto, ja
useimmat algoritmit pystyy toteuttamaan
tehokkaasti sitä käyttäen.

Kätevä tapa tallentaa verkon vieruslistaesitys on luoda taulukko,
jossa jokainen alkio on vektori:
\begin{lstlisting}
vector<int> v[N];
\end{lstlisting}

Taulukossa solmun $s$ vieruslista on kohdassa $\texttt{v}[s]$.
Vakio $N$ on valittu niin suureksi,
että kaikki vieruslistat mahtuvat taulukkoon.
Esimerkiksi verkon

\begin{center}
\begin{tikzpicture}[scale=0.9]
\node[draw, circle] (1) at (1,3) {$1$};
\node[draw, circle] (2) at (3,3) {$2$};
\node[draw, circle] (3) at (5,3) {$3$};
\node[draw, circle] (4) at (3,1) {$4$};

\path[draw,thick,->,>=latex] (1) -- (2);
\path[draw,thick,->,>=latex] (2) -- (3);
\path[draw,thick,->,>=latex] (2) -- (4);
\path[draw,thick,->,>=latex] (3) -- (4);
\path[draw,thick,->,>=latex] (4) -- (1);
\end{tikzpicture}
\end{center}
voi tallentaa seuraavasti:
\begin{lstlisting}
v[1].push_back(2);
v[2].push_back(3);
v[2].push_back(4);
v[3].push_back(4);
v[4].push_back(1);
\end{lstlisting}

Jos verkko on suuntaamaton, sen voi tallentaa samalla tavalla,
mutta silloin jokainen kaari lisätään kumpaankin suuntaan.

Painotetun verkon tapauksessa rakennetta voi laajentaa näin:

\begin{lstlisting}
vector<pair<int,int>> v[N];
\end{lstlisting}

Nyt vieruslistalla on pareja, joiden ensimmäinen kenttä on
kaaren kohdesolmu ja toinen kenttä on kaaren paino.
Esimerkiksi verkon

\begin{center}
\begin{tikzpicture}[scale=0.9]
\node[draw, circle] (1) at (1,3) {$1$};
\node[draw, circle] (2) at (3,3) {$2$};
\node[draw, circle] (3) at (5,3) {$3$};
\node[draw, circle] (4) at (3,1) {$4$};

\path[draw,thick,->,>=latex] (1) -- node[font=\small,label=above:5] {} (2);
\path[draw,thick,->,>=latex] (2) -- node[font=\small,label=above:7] {} (3);
\path[draw,thick,->,>=latex] (2) -- node[font=\small,label=left:6] {} (4);
\path[draw,thick,->,>=latex] (3) -- node[font=\small,label=right:5] {} (4);
\path[draw,thick,->,>=latex] (4) -- node[font=\small,label=left:2] {} (1);
\end{tikzpicture}
\end{center}
voi tallentaa seuraavasti:
\begin{lstlisting}
v[1].push_back({2,5});
v[2].push_back({3,7});
v[2].push_back({4,6});
v[3].push_back({4,5});
v[4].push_back({1,2});
\end{lstlisting}

Vieruslistaesityksen etuna on, että sen avulla on nopeaa selvittää,
mihin solmuihin tietystä solmusta pääsee kulkemaan.
Esimerkiksi seuraava silmukka käy läpi kaikki solmut,
joihin pääsee solmusta $s$:

\begin{lstlisting}
for (auto u : v[s]) {
    // käsittele solmu u
}
\end{lstlisting}

\subsubsection{Vierusmatriisiesitys}

\index{vierusmatriisi@vierusmatriisi}

\key{Vierusmatriisi} on kaksiulotteinen taulukko,
joka kertoo jokaisesta mahdollisesta kaaresta,
onko se mukana verkossa.
Vierusmatriisista on nopeaa tarkistaa,
onko kahden solmun välillä kaari.
Toisaalta matriisi vie paljon tilaa,
jos verkko on suuri.
Vierusmatriisi tallennetaan taulukkona

\begin{lstlisting}
int v[N][N];
\end{lstlisting}

jossa arvo $\texttt{v}[a][b]$ ilmaisee,
onko kaari solmusta $a$ solmuun $b$ mukana verkossa.
Jos kaari on mukana verkossa,
niin $\texttt{v}[a][b]=1$,
ja muussa tapauksessa $\texttt{v}[a][b]=0$.
Nyt esimerkiksi verkkoa
\begin{center}
\begin{tikzpicture}[scale=0.9]
\node[draw, circle] (1) at (1,3) {$1$};
\node[draw, circle] (2) at (3,3) {$2$};
\node[draw, circle] (3) at (5,3) {$3$};
\node[draw, circle] (4) at (3,1) {$4$};

\path[draw,thick,->,>=latex] (1) -- (2);
\path[draw,thick,->,>=latex] (2) -- (3);
\path[draw,thick,->,>=latex] (2) -- (4);
\path[draw,thick,->,>=latex] (3) -- (4);
\path[draw,thick,->,>=latex] (4) -- (1);
\end{tikzpicture}
\end{center}
vastaa seuraava vierusmatriisi:
\begin{center}
\begin{tikzpicture}[scale=0.7]
\draw (0,0) grid (4,4);
\node at (0.5,0.5) {1};
\node at (1.5,0.5) {0};
\node at (2.5,0.5) {0};
\node at (3.5,0.5) {0};
\node at (0.5,1.5) {0};
\node at (1.5,1.5) {0};
\node at (2.5,1.5) {0};
\node at (3.5,1.5) {1};
\node at (0.5,2.5) {0};
\node at (1.5,2.5) {0};
\node at (2.5,2.5) {1};
\node at (3.5,2.5) {1};
\node at (0.5,3.5) {0};
\node at (1.5,3.5) {1};
\node at (2.5,3.5) {0};
\node at (3.5,3.5) {0};
\node at (-0.5,0.5) {4};
\node at (-0.5,1.5) {3};
\node at (-0.5,2.5) {2};
\node at (-0.5,3.5) {1};
\node at (0.5,4.5) {1};
\node at (1.5,4.5) {2};
\node at (2.5,4.5) {3};
\node at (3.5,4.5) {4};
\end{tikzpicture}
\end{center}

Jos verkko on painotettu, vierusmatriisiesitystä voi 
laajentaa luontevasti niin, että matriisissa kerrotaan
kaaren paino, jos kaari on olemassa.
Tätä esitystapaa käyttäen esimerkiksi verkkoa

\begin{center}
\begin{tikzpicture}[scale=0.9]
\node[draw, circle] (1) at (1,3) {$1$};
\node[draw, circle] (2) at (3,3) {$2$};
\node[draw, circle] (3) at (5,3) {$3$};
\node[draw, circle] (4) at (3,1) {$4$};

\path[draw,thick,->,>=latex] (1) -- node[font=\small,label=above:5] {} (2);
\path[draw,thick,->,>=latex] (2) -- node[font=\small,label=above:7] {} (3);
\path[draw,thick,->,>=latex] (2) -- node[font=\small,label=left:6] {} (4);
\path[draw,thick,->,>=latex] (3) -- node[font=\small,label=right:5] {} (4);
\path[draw,thick,->,>=latex] (4) -- node[font=\small,label=left:2] {} (1);
\end{tikzpicture}
\end{center}
\begin{samepage}
vastaa seuraava vierusmatriisi:
\begin{center}
\begin{tikzpicture}[scale=0.7]
\draw (0,0) grid (4,4);
\node at (0.5,0.5) {2};
\node at (1.5,0.5) {0};
\node at (2.5,0.5) {0};
\node at (3.5,0.5) {0};
\node at (0.5,1.5) {0};
\node at (1.5,1.5) {0};
\node at (2.5,1.5) {0};
\node at (3.5,1.5) {5};
\node at (0.5,2.5) {0};
\node at (1.5,2.5) {0};
\node at (2.5,2.5) {7};
\node at (3.5,2.5) {6};
\node at (0.5,3.5) {0};
\node at (1.5,3.5) {5};
\node at (2.5,3.5) {0};
\node at (3.5,3.5) {0};
\node at (-0.5,0.5) {4};
\node at (-0.5,1.5) {3};
\node at (-0.5,2.5) {2};
\node at (-0.5,3.5) {1};
\node at (0.5,4.5) {1};
\node at (1.5,4.5) {2};
\node at (2.5,4.5) {3};
\node at (3.5,4.5) {4};
\end{tikzpicture}
\end{center}
\end{samepage}

\subsubsection{Kaarilistaesitys}

\index{kaarilista@kaarilista}

\key{Kaarilista} sisältää kaikki verkon kaaret.
Kaarilista on hyvä tapa tallentaa verkko,
jos algoritmissa täytyy käydä läpi
kaikki verkon kaaret eikä ole tarvetta
etsiä kaarta alkusolmun perusteella.

Kaarilistan voi tallentaa vektoriin
\begin{lstlisting}
vector<pair<int,int>> v;
\end{lstlisting}
jossa jokaisessa solmussa on parina kaaren
alku- ja loppusolmu.
Tällöin esimerkiksi verkon

\begin{center}
\begin{tikzpicture}[scale=0.9]
\node[draw, circle] (1) at (1,3) {$1$};
\node[draw, circle] (2) at (3,3) {$2$};
\node[draw, circle] (3) at (5,3) {$3$};
\node[draw, circle] (4) at (3,1) {$4$};

\path[draw,thick,->,>=latex] (1) -- (2);
\path[draw,thick,->,>=latex] (2) -- (3);
\path[draw,thick,->,>=latex] (2) -- (4);
\path[draw,thick,->,>=latex] (3) -- (4);
\path[draw,thick,->,>=latex] (4) -- (1);
\end{tikzpicture}
\end{center}
voi tallentaa seuraavasti:
\begin{lstlisting}
v.push_back({1,2});
v.push_back({2,3});
v.push_back({2,4});
v.push_back({3,4});
v.push_back({4,1});
\end{lstlisting}

\noindent
Painotetun verkon tapauksessa rakennetta voi laajentaa
esimerkiksi näin:
\begin{lstlisting}
vector<pair<pair<int,int>,int>> v;
\end{lstlisting}
Nyt listalla on pareja, joiden ensimmäinen jäsen
sisältää parina kaaren alku- ja loppusolmun,
ja toinen jäsen on kaaren paino.
Esimerkiksi verkon

\begin{center}
\begin{tikzpicture}[scale=0.9]
\node[draw, circle] (1) at (1,3) {$1$};
\node[draw, circle] (2) at (3,3) {$2$};
\node[draw, circle] (3) at (5,3) {$3$};
\node[draw, circle] (4) at (3,1) {$4$};

\path[draw,thick,->,>=latex] (1) -- node[font=\small,label=above:5] {} (2);
\path[draw,thick,->,>=latex] (2) -- node[font=\small,label=above:7] {} (3);
\path[draw,thick,->,>=latex] (2) -- node[font=\small,label=left:6] {} (4);
\path[draw,thick,->,>=latex] (3) -- node[font=\small,label=right:5] {} (4);
\path[draw,thick,->,>=latex] (4) -- node[font=\small,label=left:2] {} (1);
\end{tikzpicture}
\end{center}
\begin{samepage}
voi tallentaa seuraavasti:
\begin{lstlisting}
v.push_back({{1,2},5});
v.push_back({{2,3},7});
v.push_back({{2,4},6});
v.push_back({{3,4},5});
v.push_back({{4,1},2});
\end{lstlisting}
\end{samepage}