Graph Data Structure
In this tutorial, you will learn what a Graph Data Structure is. Also, you will find representations of a graph. A graph data structure is a collection of nodes that have data and are connected to other nodes. Let's try to understand this through an example. On facebook, everything is a node. That includes User, Photo, Album, Event, Group, Page, Comment, Story, Video, Link, Note...anything that has data is a node. Every relationship is an edge from one node to another. Whether you post a photo, join a group, like a page, etc., a new edge is created for that relationship.

Graph Terminology
Adjacency: A vertex is said to be adjacent to another vertex if there is an edge connecting them. Vertices 2 and 3 are not adjacent because there is no edge between them.
Path: A sequence of edges that allows you to go from vertex A to vertex B is called a path. 0-1, 1-2 and 0-2 are paths from vertex 0 to vertex 2.
Directed Graph: A graph in which an edge (u,v) doesn't necessarily mean that there is an edge (v, u) as well. The edges in such a graph are represented by arrows to show the direction of the edge.

Graph Representation
Graphs are commonly represented in two ways:
1. Adjacency Matrix An adjacency matrix is a 2D array of V x V vertices. Each row and column represent a vertex. If the value of any element a[i][j] is 1, it represents that there is an edge connecting vertex i and vertex j.
2. Adjacency List An adjacency list represents a graph as an array of linked lists. The index of the array represents a vertex and each element in its linked list represents the other vertices that form an edge with the vertex.
Graph Operations
The most common graph operations are:
Check if the element is present in the graph
Graph Traversal
Add elements(vertex, edges) to graph
Finding the path from one vertex to another