Strongly_connected _pmponents.cpp

//C++ Implementation of Kosaraju's algorithm to print all SCCs
//Strongly connected cpmponents
#include <iostream>
#include <list>
#include <stack>
using namespace std;

class Graph
{
    int V;    //No. of vertices
    list<int>* adj;    //An array of adjacency lists

    //Fills Stack with  vertices (in increasing order of finishing
    //times).The top element of stack has the maximum finishing
    //time
    void fillOrder(int v,bool visited[],stack<int> &Stack);

    //A recursive function to print DFS starting from v
    void DFSUtil(int v,bool visited[]);

public:
    Graph(int V);
    void addEdge(int v,int w);

    //The main function that finds and prints strongly connected
    //components
    void printSCCs();

    //Function that return transpose of this graph
    Graph getTranspose();
};

Graph::Graph(int V)
{
    this->V=V;
    adj=new list<int>[V];
}

// A recursive function to print DFS starting from v
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);
    } 
}

Graph Graph::getTranspose()
{
    Graph g(V);
    for(int v=0;v<V;v++)
    {
        //Recur for all the vertices adjacent to this vertex
        list<int>::iterator i;
        for(i=adj[v].begin();i!=adj[v].end();i++)
        {
            g.adj[*i].push_back(v);
        }
    }
    return g;
}

void Graph::addEdge(int v,int w)
{
    adj[v].push_back(w); 
}

void Graph::fillOrder(int v,bool visited[],stack<int> &Stack)
{
    //Mark the current node as visited and print it
    visited[v]=true;

    //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])
            fillOrder(*i,visited,Stack);

    //All vertices reachable from v are processed by now,push v
    Stack.push(v);
}

//The main function that finds and prints strongly connected
//components
void Graph::printSCCs()
{
    stack<int> Stack;

    //Mark all the vertices as not visited (For first DFS)
    bool* visited=new bool[V];
    for(int i=0;i<V;i++)
        visited[i]=false;

    //Fill vertices in stack according to their finishing times
    for(int i=0;i<V;i++)
        if(visited[i]==false)
            fillOrder(i,visited,Stack);

    //Create a reversed graph
    Graph gr=getTranspose();

    //Mark all the vertices as not visited (For second DFS)
    for(int i=0;i<V;i++)
        visited[i]=false;

    //Now process all vertices in order defined by Stack
    while(Stack.empty()==false)
    {
        //Pop a vertex from stack
        int v=Stack.top();
        Stack.pop();

        //Print Strongly connected component of the poped vertex
        if(visited[v]==false)
        {
            gr.DFSUtil(v,visited);
            cout<<endl;
        }
    }
}

// Driver program to test above functions
int main()
{
    // Create a graph given in the above diagram
    Graph g(5);
    g.addEdge(1, 0);
    g.addEdge(0, 2);
    g.addEdge(2, 1);
    g.addEdge(0, 3);
    g.addEdge(3, 4);

    cout << "Following are strongly connected components in "
            "given graph \n";
    g.printSCCs();

    return 0;
}