C++ Program to Find Whether a Path Exists Between 2 Given Nodes

Code:

#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 *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 [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 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::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;
}


Output:

Enter the source and destination vertices: (0-3)
1 3

There is a path from 1 to 3
There is no path from 3 to 1

Enter the source and destination vertices: (0-3)
2 3

There is a path from 2 to 3
There is no path from 3 to 2

------------------
(program exited with code: 0)
Press return to continue



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