C++ Program to Represent Graph Using Incidence List

This program represents a graph using incidence list and the time complexity of this algorithm is O(e).

Algorithm

Begin
   Take the input of the number of vertex ‘v’ and edges ‘e’ and also
   take the input of ‘e’ pairs of vertexes of the given graph in e[][].
   For each edge print the corresponding vertex involved in that connection.
End

Example Code

#include<iostream>
using namespace std;
int main() {
   int i, v, e, j, c;
   cout<<"Enter the number of vertexes of the graph: ";
   cin>>v;
   cout<<"\nEnter the number of edges of the graph: ";
   cin>>e;
   int edge[e][2];
   for(i = 0; i < e; i++) {
      cout<<"\nEnter the vertex pair for edge "<<i+1;
      cout<<"\nV(1): ";
      cin>>edge[i][0];
      cout<<"V(2): ";
      cin>>edge[i][1];
   }
   cout<<"\n\nThe incidence list representation for the given graph: ";
   for(i = 0; i < e; i++) {
      // For each vertex print, its adjacent vertex.
      cout<<"\n\tE("<<i+1<<") -> { ";
         cout<<"V("<<edge[i][0]<<") , "<<"V("<<edge[i][1]<<")";
         cout<<" }";
   }
}

Output

Enter the number of vertexes of the graph: 3
Enter the number of edges of the graph: 4
Enter the vertex pair for edge 1
V(1): 2
V(2): 1
Enter the vertex pair for edge 2
V(1): 1
V(2): 2
Enter the vertex pair for edge 3
V(1): 3
V(2): 2
Enter the vertex pair for edge 4
V(1): 2
V(2): 3
The incidence list representation for the given graph:
E(1) -> { V(2) , V(1) }
E(2) -> { V(1) , V(2) }
E(3) -> { V(3) , V(2) }
E(4) -> { V(2) , V(3) }