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Program to Find Whether a Path Exists Between 2 Given Nodes



Program to Find Whether a Path Exists Between 2 Given Nodes

This is a C++ Program to check whether path exists between two given nodes. The idea is to run the depth first search algorithm with the given source node, if during dfs we visit destination node, path exists, not otherwise.

    #include <iostream>

    #include <list>

    using namespace std;

    // This class represents a directed graph using adjacency list representation

    class Graph

    {

            int V; // No. of vertices

            list<int> *adj; // Pointer to an array containing adjacency lists

        public:

            Graph(int V); // Constructor

            void addEdge(int v, int w); // function to add an edge to graph

            bool isReachable(int s, int d); // returns true if there is a path from s to d

    };

    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 v's list.

    }

    // A BFS based function to check whether d is reachable from s.

    bool Graph::isReachable(int s, int d)

    {

        // Base case

        if (s == d)

            return true;

        // Mark all the vertices as not visited

        bool *visited = new bool[V];

        for (int i = 0; i < V; i++)

            visited[i] = false;

        // Create a queue for BFS

        list<int> queue;

        // Mark the current node as visited and enqueue it

        visited[s] = true;

        queue.push_back(s);

        // it will be used to get all adjacent vertices of a vertex

        list<int>::iterator i;

        while (!queue.empty())

        {

            // Dequeue a vertex from queue and print it

            s = queue.front();

            queue.pop_front();

            // Get all adjacent vertices of the dequeued vertex s

            // If a adjacent has not been visited, then mark it visited

            // and enqueue it

            for (i = adj[s].begin(); i != adj[s].end(); ++i)

            {

                // If this adjacent node is the destination node, then return true

                if (*i == d)

                    return true;

                // Else, continue to do BFS

                if (!visited[*i])

                {

                    visited[*i] = true;

                    queue.push_back(*i);

                }

            }

        }

        return false;

    }

    // Driver program to test methods of graph class

    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 << "Enter the source and destination vertices: (0-3)";

        int u, v;

        cin >> u >> v;

        if (g.isReachable(u, v))

            cout << "\nThere is a path from " << u << " to " << v;

        else

            cout << "\nThere is no path from " << u << " to " << v;

        int temp;

        temp = u;

        u = v;

        v = temp;

        if (g.isReachable(u, v))

            cout << "\nThere is a path from " << u << " to " << v;

        else

            cout << "\nThere is no path from " << u << " to " << v;

        return 0;

    }