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#include <iostream>
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#include <list>
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using namespace std;
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// This class represents a directed graph using adjacency list representation
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class Graph
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{
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int V; // No. of vertices
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list<int> *adj; // Pointer to an array containing adjacency lists
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public:
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Graph(int V); // Constructor
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void addEdge(int v, int w); // function to add an edge to graph
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bool isReachable(int s, int d); // returns true if there is a path from s to d
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};
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Graph::Graph(int V)
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{
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this->V = V;
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adj = new list<int> [V];
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}
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void Graph::addEdge(int v, int w)
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{
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adj[v].push_back(w); // Add w to v’s list.
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}
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// A BFS based function to check whether d is reachable from s.
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bool Graph::isReachable(int s, int d)
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{
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// Base case
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if (s == d)
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return true;
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// Mark all the vertices as not visited
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bool *visited = new bool[V];
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for (int i = 0; i < V; i++)
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visited[i] = false;
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// Create a queue for BFS
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list<int> queue;
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// Mark the current node as visited and enqueue it
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visited[s] = true;
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queue.push_back(s);
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// it will be used to get all adjacent vertices of a vertex
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list<int>::iterator i;
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while (!queue.empty())
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{
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// Dequeue a vertex from queue and print it
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s = queue.front();
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queue.pop_front();
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// Get all adjacent vertices of the dequeued vertex s
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// If a adjacent has not been visited, then mark it visited
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// and enqueue it
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for (i = adj[s].begin(); i != adj[s].end(); ++i)
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{
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// If this adjacent node is the destination node, then return true
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if (*i == d)
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return true;
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// Else, continue to do BFS
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if (!visited[*i])
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{
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visited[*i] = true;
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queue.push_back(*i);
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}
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}
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}
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return false;
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}
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// Driver program to test methods of graph class
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int main()
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{
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// Create a graph given in the above diagram
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Graph g(4);
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g.addEdge(0, 1);
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g.addEdge(0, 2);
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g.addEdge(1, 2);
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g.addEdge(2, 0);
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g.addEdge(2, 3);
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g.addEdge(3, 3);
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cout << "Enter the source and destination vertices: (0-3)";
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int u, v;
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cin >> u >> v;
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if (g.isReachable(u, v))
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cout << "\nThere is a path from " << u << " to " << v;
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else
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cout << "\nThere is no path from " << u << " to " << v;
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int temp;
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temp = u;
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u = v;
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v = temp;
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if (g.isReachable(u, v))
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cout << "\nThere is a path from " << u << " to " << v;
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else
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cout << "\nThere is no path from " << u << " to " << v;
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return 0;
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}
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/*
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Enter the source and destination vertices: (0-3)
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1 3
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There is a path from 1 to 3
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There is no path from 3 to 1
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Enter the source and destination vertices: (0-3)
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2 3
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There is a path from 2 to 3
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There is no path from 3 to 2
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