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Java Program for Depth First Search or DFS for a Graph

  • Last Updated : 18 May, 2022

Depth First Traversal (or Search) for a graph is similar to Depth First Traversal of a tree. The only catch here is, unlike trees, graphs may contain cycles, so we may come to the same node again. To avoid processing a node more than once, we use a boolean visited array.

For example, in the following graph, we start traversal from vertex 2. When we come to vertex 0, we look for all adjacent vertices of it. 2 is also an adjacent vertex of 0. If we don\’t mark visited vertices, then 2 will be processed again and it will become a non-terminating process. A Depth First Traversal of the following graph is 2, 0, 1, 3.

See this post for all applications of Depth First Traversal.
Following are implementations of simple Depth First Traversal. The C++ implementation uses adjacency list representation of graphs. STL\’s list container is used to store lists of adjacent nodes.

Recommended: Please solve it on “PRACTICE ” first, before moving on to the solution.

Java




// Java program to print DFS traversal from a given graph
import java.io.*;
import java.util.*;
  
// This class represents a directed graph using adjacency list
// representation
class Graph
{
    private int V;   // No. of vertices
  
    // Array  of lists for Adjacency List Representation
    private LinkedList<Integer> adj[];
  
    // Constructor
    Graph(int v)
    {
        V = v;
        adj = new LinkedList[v];
        for (int i=0; i<v; ++i)
            adj[i] = new LinkedList();
    }
  
    //Function to add an edge into the graph
    void addEdge(int v, int w)
    {
        adj[v].add(w);    // Add w to v\'s list.
    }
  
    // A function used by DFS
    void DFSUtil(int v,boolean visited[])
    {
        // Mark the current node as visited and print it
        visited[v] = true;
        System.out.print(v+" ");
  
        // Recur for all the vertices adjacent to this vertex
        Iterator<Integer> i = adj[v].listIterator();
        while (i.hasNext())
        {
            int n = i.next();
            if (!visited[n])
                DFSUtil(n,visited);
        }
    }
  
    // The function to do DFS traversal. It uses recursive DFSUtil()
    void DFS()
    {
        // Mark all the vertices as not visited(set as
        // false by default in java)
        boolean visited[] = new boolean[V];
  
        // Call the recursive helper function to print DFS traversal
        // starting from all vertices one by one
        for (int i=0; i<V; ++i)
            if (visited[i] == false)
                DFSUtil(i, visited);
    }
  
    public static void main(String args[])
    {
        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("Following is Depth First Traversal");
  
        g.DFS();
    }
}
// This code is contributed by Aakash Hasija

Please refer complete article on Depth First Search or DFS for a Graph for more details!


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