Convert Adjacency List to Adjacency Matrix representation of a Graph

Given an adjacency list representation of a Graph, the task is to convert the given Adjacency List to Adjacency Matrix representation.
Examples: 

Input: adjList[] = {{0 –> 1 –> 3}, {1 –> 2}, {2 –> 3}} 
Output: 
0 1 0 1
0 0 1 0
0 0 0 1
0 0 0 0

Input: adjList[] = {{0 –> 1 –> 4}, {1 –> 0 –> 2 –> 3 –> 4}, {2 –> 1 –> 3}, {3 –> 1 –> 2 –> 4}, {4 –> 0 –> 1 –> 3}} 
Output: 
0 1 0 0 1
1 0 1 1 1
0 1 0 1 0
0 1 1 0 1
1 1 0 1 0 

Adjacency List: An array of lists is used. The size of the array is equal to the number of vertices. Let the array be an array[]. An entry array[i] represents the list of vertices adjacent to the ith Vertex.

Adjacency Matrix: Adjacency Matrix is a 2D array of size V x V where V is the number of vertices in a graph. Let the 2D array be adj[][], a slot adj[i][j] = 1 indicates that there is an edge from vertex i to vertex j.



Follow the steps below to convert an adjacency list to an adjacency matrix: 

  • Initialize a matrix with 0s.
  • Iterate over the vertices in the adjacency list
  • For every jth vertex in the adjacency list, traverse its edges.
  • For each vertex i with which the jth vertex has an edge, set mat[i][j] = 1.

Below is the implementation of the above approach:

C++

filter_none

edit
close

play_arrow

link
brightness_4
code

// C++ Program to implement
// the above approach
#include <bits/stdc++.h>
using namespace std;
  
// Function to insert vertices to adjacency list
void insert(vector<int> adj[], int u, int v)
{
    // Insert a vertex v to vertex u
    adj[u].push_back(v);
    return;
}
  
// Function to display adjacency list
void printList(vector<int> adj[], int V)
{
    for (int i = 0; i < V; i++) {
        cout << i;
        for (auto j : adj[i])
            cout << " --> " << j;
        cout << endl;
    }
    cout << endl;
}
  
// Function to convert adjacency
// list to adjacency matrix
vector<vector<int> > convert(vector<int> adj[],
                             int V)
{
    // Initialize a matrix
    vector<vector<int> > matrix(V,
                                vector<int>(V, 0));
  
    for (int i = 0; i < V; i++) {
        for (auto j : adj[i])
            matrix[i][j] = 1;
    }
    return matrix;
}
  
// Function to display adjacency matrix
void printMatrix(vector<vector<int> > adj, int V)
{
    for (int i = 0; i < V; i++) {
        for (int j = 0; j < V; j++) {
            cout << adj[i][j] << "   ";
        }
        cout << endl;
    }
    cout << endl;
}
  
// Driver code
int main()
{
    int V = 5;
  
    vector<int> adjList[V];
  
    // Inserting edges
    insert(adjList, 0, 1);
    insert(adjList, 0, 4);
    insert(adjList, 1, 0);
    insert(adjList, 1, 2);
    insert(adjList, 1, 3);
    insert(adjList, 1, 4);
    insert(adjList, 2, 1);
    insert(adjList, 2, 3);
    insert(adjList, 3, 1);
    insert(adjList, 3, 2);
    insert(adjList, 3, 4);
    insert(adjList, 4, 0);
    insert(adjList, 4, 1);
    insert(adjList, 4, 3);
  
    // Display adjacency list
    cout << "Adjacency List: \n";
    printList(adjList, V);
  
    // Function call which returns
    // adjacency matrix after conversion
    vector<vector<int> > adjMatrix
        = convert(adjList, V);
  
    // Display adjacency matrix
    cout << "Adjacency Matrix: \n";
    printMatrix(adjMatrix, V);
  
    return 0;
}

chevron_right


Java

filter_none

edit
close

play_arrow

link
brightness_4
code

// Java program to implement
// the above approach
import java.util.*;
  
class GFG{
  
// Function to insert vertices to adjacency list
static void insert(Vector<Integer> adj[],
                   int u, int v)
{
      
    // Insert a vertex v to vertex u
    adj[u].add(v);
    return;
}
  
// Function to display adjacency list
static void printList(Vector<Integer> adj[], 
                      int V)
{
    for(int i = 0; i < V; i++)
    {
        System.out.print(i);
  
        for(int j : adj[i])
            System.out.print(" --> " + j);
              
        System.out.println();
    }
    System.out.println();
}
  
// Function to convert adjacency
// list to adjacency matrix
static int[][] convert(Vector<Integer> adj[],
                       int V)
{
      
    // Initialize a matrix
    int [][]matrix = new int[V][V];
  
    for(int i = 0; i < V; i++) 
    {
        for(int j : adj[i])
            matrix[i][j] = 1;
    }
    return matrix;
}
  
// Function to display adjacency matrix
static void printMatrix(int[][] adj, int V)
{
    for(int i = 0; i < V; i++) 
    {
        for(int j = 0; j < V; j++)
        {
            System.out.print(adj[i][j] + " ");
        }
        System.out.println();
    }
    System.out.println();
}
  
// Driver code
public static void main(String[] args)
{
    int V = 5;
  
    @SuppressWarnings("unchecked")
    Vector<Integer> []adjList = new Vector[V];
    for(int i = 0; i < adjList.length; i++)
        adjList[i] = new Vector<Integer>();
          
    // Inserting edges
    insert(adjList, 0, 1);
    insert(adjList, 0, 4);
    insert(adjList, 1, 0);
    insert(adjList, 1, 2);
    insert(adjList, 1, 3);
    insert(adjList, 1, 4);
    insert(adjList, 2, 1);
    insert(adjList, 2, 3);
    insert(adjList, 3, 1);
    insert(adjList, 3, 2);
    insert(adjList, 3, 4);
    insert(adjList, 4, 0);
    insert(adjList, 4, 1);
    insert(adjList, 4, 3);
  
    // Display adjacency list
    System.out.print("Adjacency List: \n");
    printList(adjList, V);
  
    // Function call which returns
    // adjacency matrix after conversion
    int[][] adjMatrix = convert(adjList, V);
  
    // Display adjacency matrix
    System.out.print("Adjacency Matrix: \n");
    printMatrix(adjMatrix, V);
}
}
  
// This code is contributed by amal kumar choubey

chevron_right


C#

filter_none

edit
close

play_arrow

link
brightness_4
code

// C# program to implement
// the above approach
using System;
using System.Collections.Generic;
  
class GFG{
  
// Function to insert vertices to adjacency list
static void insert(List<int> []adj,
                        int u, int v)
{
      
    // Insert a vertex v to vertex u
    adj[u].Add(v);
    return;
}
  
// Function to display adjacency list
static void printList(List<int> []adj, 
                           int V)
{
    for(int i = 0; i < V; i++)
    {
        Console.Write(i);
  
        foreach(int j in adj[i])
            Console.Write(" --> " + j);
              
        Console.WriteLine();
    }
    Console.WriteLine();
}
  
// Function to convert adjacency
// list to adjacency matrix
static int[,] convert(List<int> []adj,
                           int V)
{
      
    // Initialize a matrix
    int [,]matrix = new int[V, V];
  
    for(int i = 0; i < V; i++) 
    {
        foreach(int j in adj[i])
            matrix[i, j] = 1;
    }
    return matrix;
}
  
// Function to display adjacency matrix
static void printMatrix(int[,] adj, int V)
{
    for(int i = 0; i < V; i++) 
    {
        for(int j = 0; j < V; j++)
        {
            Console.Write(adj[i, j] + " ");
        }
        Console.WriteLine();
    }
    Console.WriteLine();
}
  
// Driver code
public static void Main(String[] args)
{
    int V = 5;
  
    List<int> []adjList = new List<int>[V];
    for(int i = 0; i < adjList.Length; i++)
        adjList[i] = new List<int>();
          
    // Inserting edges
    insert(adjList, 0, 1);
    insert(adjList, 0, 4);
    insert(adjList, 1, 0);
    insert(adjList, 1, 2);
    insert(adjList, 1, 3);
    insert(adjList, 1, 4);
    insert(adjList, 2, 1);
    insert(adjList, 2, 3);
    insert(adjList, 3, 1);
    insert(adjList, 3, 2);
    insert(adjList, 3, 4);
    insert(adjList, 4, 0);
    insert(adjList, 4, 1);
    insert(adjList, 4, 3);
  
    // Display adjacency list
    Console.Write("Adjacency List: \n");
    printList(adjList, V);
  
    // Function call which returns
    // adjacency matrix after conversion
    int[,] adjMatrix = convert(adjList, V);
  
    // Display adjacency matrix
    Console.Write("Adjacency Matrix: \n");
    printMatrix(adjMatrix, V);
}
}
  
// This code is contributed by amal kumar choubey

chevron_right


Output: 

Adjacency List: 
0 --> 1 --> 4
1 --> 0 --> 2 --> 3 --> 4
2 --> 1 --> 3
3 --> 1 --> 2 --> 4
4 --> 0 --> 1 --> 3

Adjacency Matrix: 
0   1   0   0   1   
1   0   1   1   1   
0   1   0   1   0   
0   1   1   0   1   
1   1   0   1   0

Time Complexity: O(N*M) 
Auxiliary Space: O(N2)
 

Attention reader! Don’t stop learning now. Get hold of all the important DSA concepts with the DSA Self Paced Course at a student-friendly price and become industry ready.




My Personal Notes arrow_drop_up

Check out this Author's contributed articles.

If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

Please Improve this article if you find anything incorrect by clicking on the "Improve Article" button below.



Improved By : Amal Kumar Choubey