After the short introduction, you know some basic information about graphs, especially regarding nodes and various kinds of edges. However, why is the topic of graphs so important and why does it take up a whole chapter in this book? Could you use this data structure in your applications? The answer is obvious: yes! The graphs are commonly used while solving various algorithmic problems and have numerous real-world applications. Two examples are shown in the following diagrams.

To start with, let's think about a structure of friends available in social media. Each user has many contacts, but they also have many friends, and so on. What data structure should you choose to store such data? The graph is one of the simplest answers. In such a scenario, the nodes represent contacts, while edges depict relationships between people. As an example, let's take a look at the following diagram of an undirected and unweighted graph:

As you can see, Jimmy Stewart has five contacts, namely...