Transitive Closure of a Graph using DFS

Given a directed graph, find out if a vertex v is reachable from another vertex u for all vertex pairs (u, v) in the given graph. Here reachable means that there is a path from vertex u to v. The reach-ability matrix is called transitive closure of a graph.

For example, consider below graph

Transitive closure of above graphs is 
     1 1 1 1 
     1 1 1 1 
     1 1 1 1 
     0 0 0 1 

We have discussed an O(V³) solution for this here. The solution was based on Floyd Warshall Algorithm. In this post, an O(V(V+E)) algorithm for the same is discussed. So for dense graph, it would become O(V³) and for sparse graph, it would become O(V²).

Below are the abstract steps of the algorithm. 

1. Create a matrix tc[V][V] that would finally have transitive closure of the given graph. Initialize all entries of tc[][] as 0.
2. Call DFS for every node of the graph to mark reachable vertices in tc[][]. In recursive calls to DFS, we don’t call DFS for an adjacent vertex if it is already marked as reachable in tc[][].

Below is the implementation of the above idea. The code uses adjacency list representation of input graph and builds a matrix tc[V][V] such that tc[u][v] would be true if v is reachable from u.

Implementation:

C++

// C++ program to print transitive closure of a graph
#include <bits/stdc++.h>
using namespace std;

class Graph {
    int V; // No. of vertices
    bool** tc; // To store transitive closure
    list<int>* adj; // array of adjacency lists
    void DFSUtil(int u, int v);

public:
    Graph(int V); // Constructor

    // function to add an edge to graph
    void addEdge(int v, int w) { adj[v].push_back(w); }

    // prints transitive closure matrix
    void transitiveClosure();
};

Graph::Graph(int V)
{
    this->V = V;
    adj = new list<int>[V];

    tc = new bool*[V];
    for (int i = 0; i < V; i++) {
        tc[i] = new bool[V];
        memset(tc[i], false, V * sizeof(bool));
    }
}

// A recursive DFS traversal function that finds
// all reachable vertices for s.
void Graph::DFSUtil(int s, int v)
{
    // Mark reachability from s to t as true.
    if (s == v) {
        tc[s][v] = true;
    }
    else
        tc[s][v] = true;

    // Find all the vertices reachable through v
    list<int>::iterator i;
    for (i = adj[v].begin(); i != adj[v].end(); ++i) {
        if (tc[s][*i] == false) {
            if (*i == s) {
                tc[s][*i] = 1;
            }
            else {
                DFSUtil(s, *i);
            }
        }
    }
}

// The function to find transitive closure. It uses
// recursive DFSUtil()
void Graph::transitiveClosure()
{
    // Call the recursive helper function to print DFS
    // traversal starting from all vertices one by one
    for (int i = 0; i < V; i++)
        DFSUtil(i,
                i); // Every vertex is reachable from self.

    for (int i = 0; i < V; i++) {
        for (int j = 0; j < V; j++)
            cout << tc[i][j] << " ";
        cout << endl;
    }
}

// Driver code
int main()
{

    // Create a graph given in the above diagram
    Graph g(4);
    g.addEdge(0, 1);
    g.addEdge(0, 2);
    g.addEdge(1, 2);
    g.addEdge(2, 0);
    g.addEdge(2, 3);
    g.addEdge(3, 3);
    cout << "Transitive closure matrix is \n";
    g.transitiveClosure();
    return 0;
}

Java

// JAVA program to print transitive
// closure of a graph.

import java.util.ArrayList;
import java.util.Arrays;

// A directed graph using
// adjacency list representation
public class Graph {

        // No. of vertices in graph
    private int vertices;

        // adjacency list
    private ArrayList<Integer>[] adjList;

        // To store transitive closure
    private int[][] tc;

    // Constructor
    public Graph(int vertices) {

             // initialise vertex count
             this.vertices = vertices;
             this.tc = new int[this.vertices][this.vertices];

             // initialise adjacency list
             initAdjList();
    }

    // utility method to initialise adjacency list
    @SuppressWarnings("unchecked")
    private void initAdjList() {

        adjList = new ArrayList[vertices];
        for (int i = 0; i < vertices; i++) {
            adjList[i] = new ArrayList<>();
        }
    }

    // add edge from u to v
    public void addEdge(int u, int v) {

      // Add v to u's list.
        adjList[u].add(v);
    }

    // The function to find transitive
    // closure. It uses
    // recursive DFSUtil()
    public void transitiveClosure() {

        // Call the recursive helper
        // function to print DFS
        // traversal starting from all
        // vertices one by one
        for (int i = 0; i < vertices; i++) {
            dfsUtil(i, i);
        }

        for (int i = 0; i < vertices; i++) {
          System.out.println(Arrays.toString(tc[i]));
        }
    }

    // A recursive DFS traversal
    // function that finds
    // all reachable vertices for s
    private void dfsUtil(int s, int v) {

        // Mark reachability from
        // s to v as true.
       if(s==v){
          tc[s][v] = 1;
       }
      else
        tc[s][v] = 1;

        // Find all the vertices reachable
        // through v
        for (int adj : adjList[v]) {
            if (tc[s][adj]==0) {
                dfsUtil(s, adj);
            }
        }
    }

    // Driver Code
    public static void main(String[] args) {

        // Create a graph given
        // in the above diagram
        Graph g = new Graph(4);

        g.addEdge(0, 1);
        g.addEdge(0, 2);
        g.addEdge(1, 2);
        g.addEdge(2, 0);
        g.addEdge(2, 3);
        g.addEdge(3, 3);
        System.out.println("Transitive closure " +
                "matrix is");

        g.transitiveClosure();

    }
}


// This code is contributed
// by Himanshu Shekhar

Python3

# Python program to print transitive
# closure of a graph.
from collections import defaultdict

class Graph:

    def __init__(self,vertices):
        # No. of vertices
        self.V = vertices

        # default dictionary to store graph
        self.graph = defaultdict(list)

        # To store transitive closure
        self.tc = [[0 for j in range(self.V)] for i in range(self.V)]

    # function to add an edge to graph
    def addEdge(self, u, v):
        self.graph[u].append(v)

    # A recursive DFS traversal function that finds
    # all reachable vertices for s
    def DFSUtil(self, s, v):

        # Mark reachability from s to v as true.
        if(s == v):
            if( v in self.graph[s]):
              self.tc[s][v] = 1
        else:
            self.tc[s][v] = 1

        # Find all the vertices reachable through v
        for i in self.graph[v]:
            if self.tc[s][i] == 0:
                if s==i:
                   self.tc[s][i]=1
                else:
                   self.DFSUtil(s, i)

    # The function to find transitive closure. It uses
    # recursive DFSUtil()
    def transitiveClosure(self):

        # Call the recursive helper function to print DFS
        # traversal starting from all vertices one by one
        for i in range(self.V):
            self.DFSUtil(i, i)

        print(self.tc)

# Create a graph given in the above diagram
g = Graph(4)
g.addEdge(0, 1)
g.addEdge(0, 2)
g.addEdge(1, 2)
g.addEdge(2, 0)
g.addEdge(2, 3)
g.addEdge(3, 3)

g.transitiveClosure()

C#

// C# program to print transitive
// closure of a graph.
using System;
using System.Collections.Generic;

// A directed graph using
// adjacency list representation
public class Graph {

    // No. of vertices in graph
    private int vertices;

    // adjacency list
    private List<int>[] adjList;

    // To store transitive closure
    private int[, ] tc;

    // Constructor
    public Graph(int vertices)
    {

        // initialise vertex count
        this.vertices = vertices;
        this.tc = new int[this.vertices, this.vertices];

        // initialise adjacency list
        initAdjList();
    }

    // utility method to initialise adjacency list
    private void initAdjList()
    {

        adjList = new List<int>[ vertices ];
        for (int i = 0; i < vertices; i++) {
            adjList[i] = new List<int>();
        }
    }

    // add edge from u to v
    public void addEdge(int u, int v)
    {

        // Add v to u's list.
        adjList[u].Add(v);
    }

    // The function to find transitive
    // closure. It uses
    // recursive DFSUtil()
    public void transitiveClosure()
    {

        // Call the recursive helper
        // function to print DFS
        // traversal starting from all
        // vertices one by one
        for (int i = 0; i < vertices; i++) {
            dfsUtil(i, i);
        }

        for (int i = 0; i < vertices; i++) {
            for (int j = 0; j < vertices; j++)
                Console.Write(tc[i, j] + " ");
            Console.WriteLine();
        }
    }

    // A recursive DFS traversal
    // function that finds
    // all reachable vertices for s
    private void dfsUtil(int s, int v)
    {

        // Mark reachability from
        // s to v as true.
        tc[s, v] = 1;

        // Find all the vertices reachable
        // through v
        foreach(int adj in adjList[v])
        {
            if (tc[s, adj] == 0) {
                dfsUtil(s, adj);
            }
        }
    }

    // Driver Code
    public static void Main(String[] args)
    {

        // Create a graph given
        // in the above diagram
        Graph g = new Graph(4);
        g.addEdge(0, 1);
        g.addEdge(0, 2);
        g.addEdge(1, 2);
        g.addEdge(2, 0);
        g.addEdge(2, 3);
        g.addEdge(3, 3);
        Console.WriteLine("Transitive closure "
                          + "matrix is");
        g.transitiveClosure();
    }
}

// This code is contributed by Rajput-Ji

Javascript

<script>
/* Javascript program to print transitive
 closure of a graph*/

    class Graph
    {
        // Constructor
        constructor(v)
        {
            this.V = v;
            this.adj = new Array(v);
            this.tc = Array.from(Array(v), () => new Array(v).fill(0));
            for(let i = 0; i < v; i++)
                this.adj[i] = [];
        }

        // function to add an edge to graph
        addEdge(v, w)
        {
            this.adj[v].push(w);
        }

        // A recursive DFS traversal function that finds
        // all reachable vertices for s.
        DFSUtil(s, v)
        {
            // Mark reachability from s to v as true.
            this.tc[s][v] = 1;

            // Find all the vertices reachable through v
            for(let i of this.adj[v].values())
            {
                if(this.tc[s][i] == 0)
                    this.DFSUtil(s, i);
            }
        }

        // The function to find transitive closure. It uses
        // recursive DFSUtil()
        transitiveClosure()
        {
            // Call the recursive helper function to print DFS
            // traversal starting from all vertices one by one
            for(let i = 0; i < this.V; i++)
                this.DFSUtil(i, i); // Every vertex is reachable from self

            document.write("Transitive closure matrix is<br />")
            for(let i=0; i < this.V; i++)
            {
                for(let j=0; j < this.V; j++)
                    document.write(this.tc[i][j] + " ");
                document.write("<br />")
            }
        }

    };

    // driver code
    g = new Graph(4);
    g.addEdge(0, 1);
    g.addEdge(0, 2);
    g.addEdge(1, 2);
    g.addEdge(2, 0);
    g.addEdge(2, 3);
    g.addEdge(3, 3);

    g.transitiveClosure();

    // This code is contributed by cavi4762.
</script>

Output

Transitive closure matrix is 
1 1 1 1 
1 1 1 1 
1 1 1 1 
0 0 0 1 

Time Complexity : O(V^2) where V is the number of vertexes .

Space complexity : O(V^2) where V is number of vertices.