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C Program for Depth First Search or DFS for a Graph

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  • Last Updated : 20 Dec, 2021

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 ā€œPRACTICE ā€ first, before moving on to the solution.

C++




// C++ program to print DFS traversal for a given given graph
#include<iostream>
#include         <list>
using namespace std;
  
class Graph
{
    int V;    // No. of vertices
    list<int> *adj;    // Pointer to an array containing adjacency lists
    void DFSUtil(int v, bool visited[]);  // A function used by DFS
public:
    Graph(int V);   // Constructor
    void addEdge(int v, int w);   // function to add an edge to graph
    void DFS();    // prints DFS traversal of the complete graph
};
  
Graph::Graph(int V)
{
    this->V = V;
    adj = new list<int>[V];
}
  
void Graph::addEdge(int v, int w)
{
    adj[v].push_back(w); // Add w to vā€™s list.
}
  
void Graph::DFSUtil(int v, bool visited[])
{
    // Mark the current node as visited and print it
    visited[v] = true;
    cout << v << " ";
  
    // Recur for all the vertices adjacent to this vertex
    list<int>::iterator i;
    for(i = adj[v].begin(); i != adj[v].end(); ++i)
        if(!visited[*i])
            DFSUtil(*i, visited);
}
  
// The function to do DFS traversal. It uses recursive DFSUtil()
void Graph::DFS()
{
    // Mark all the vertices as not visited
    bool *visited = new bool[V];
    for (int i = 0; i < V; i++)
        visited[i] = false;
  
    // 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);
}
  
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 << "Following is Depth First Traversaln";
    g.DFS();
  
    return 0;
}

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


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