** Adjacency List**

In the * adjacency list* representation of
a graph, each node has a list of nodes that are * adjacent*
to it, i.e. connected by an edge. A linked list is
a natural representation.

1 | (5 2) |

2 | (3 5 1) |

3 | (4 2) |

4 | (6 3 5) |

5 | (4 2 1) |

6 | (4) |

This graph is undirected, so each link is represented twice.

The storage required is *O(|V| + |E|)*. This is a good representation if
the graph is * sparse*, i.e. each node is not linked
to many others.