added a graph algo

Author: hardikhere

Graph/Strongly Connected Components.cpp

@@ -0,0 +1,108 @@
+// A directed graph is strongly connected if 
+// there is a path between all pairs of vertices. 
+// A strongly connected component (SCC) of a directed graph 
+// is a maximal strongly connected subgraph.
+#include<bits/stdc++.h>
+using namespace std;
+
+class Graph{
+    public:
+     int V;
+     list<int> *adj;
+     Graph(int V);
+     void addEdge(int u,int v);
+     void fillFinishtime(int v,bool visited[],stack<int>&Stack);
+     void DFSUtil(int v,bool visited[]);
+     void printSCCs();
+     Graph getTranspose();
+};
+
+Graph Graph::getTranspose(){
+    Graph trans(V);
+    for(int v=0;v<V;v++){
+        list<int>::iterator i;
+        for(i=adj[v].begin();i!=adj[v].end();++i){
+            trans.adj[*i].push_back(v);
+        }
+    }
+    return trans;
+}
+
+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); 
+}
+
+
+void Graph::fillFinishtime(int v,bool visited[],stack<int>&Stack){
+        visited[v] = true;
+        list<int> ::iterator it;
+        for(it = adj[v].begin();it!=adj[v].end();++it)
+            if(!visited[*it])
+               fillFinishtime(*it,visited,Stack);
+        Stack.push(v);
+}
+
+void Graph:: printSCCs(){
+    stack<int>Stack;
+    bool *visited = new bool[V];
+    for(int i=0;i<V;i++)
+       visited[i] = false;
+    //fill vertices in stack accd to their finish times
+    for(int i=0;i<V;i++)
+        if(!visited[i])
+            fillFinishtime(i,visited,Stack);
+    //create reversed graph
+    // cout<<"stack is \n";
+    // cout<<"size of the stack is "<<Stack.size()<<endl;
+    // while(!Stack.empty())
+    //    {cout<<Stack.top()<<endl;Stack.pop();}
+    // cout<<endl;
+    Graph reversed = getTranspose();
+   // marke all unvisited for 2nd dfs
+    for(int i=0;i<V;i++)
+        visited[i] = false;
+    while(!Stack.empty()){
+        int v = Stack.top();
+        Stack.pop();
+        if(!visited[v]){
+            reversed.DFSUtil(v,visited);
+            cout<<endl;
+        }
+    }
+
+}
+
+
+void Graph::addEdge(int u, int v){
+    adj[u].push_back(v);
+}
+Graph::Graph(int v){
+    this->V = v;
+    adj = new list<int>[V];
+}
+
+
+int main(){
+    Graph g(5); g.addEdge(0, 2);
+    g.addEdge(1, 0); 
+     
+    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;
+  
+}