Introduction to Graphs

Note: Graphs will become a key topic in higher divisions. For Bronze, graphs are just a nice way to think about the structure of our data.

Graphs can be used to represent many things, from images to wireless signals, but one of the simplest analogies is to a map. Consider a map with several cities and bidirectional roads connecting the cities. Some problems relating to graphs are:

1. Is city $A$ connected to city $B$? Consider a region to be a group of cities such that each city in the group can reach any other city in said group, but no other cities. How many regions are in this map, and which cities are in which region? (Silver)

2. What's the shortest distance I have to travel to get from city $A$ to city $B$? (Gold)

For now, it suffices to learn how graphs are represented (usually adjacency lists).

What Does a Bronze Graph Problem Look Like?

All of the problems below fall into at least one of the following two categories:

• The graph's structure is special (it's a tree, path, or a cycle).
• To solve the problem, all you need to do is iterate over the adjacency list of every vertex.

Knowing DFS can be helpful but it should not be required.

Problems