Graph:

Vertex + edge (neighbors).

Representation:

Vertex 1) state 2) entity

Representation:

1) adj matrix

space : O(v ^ 2)

pros : look up for pair O(1). cons : sparse E << V

2) adj list

class Vertex {
    int id;
    List<Vertex> neighbors;
}

class Graph {
    List<Vertex> vertices;
}

Sparse Table: