add kosaraju algorithm in cpp (jainaman224#2672)

* add kosaraju algorithm in cpp

* Update kosaraju_algorithm.cpp

kosaraju_algorithm/kosaraju_algorithm.cpp

@@ -0,0 +1,105 @@
+//Strongly connected components(KOSARAJU ALGORITHM)
+#include <iostream>
+#include <vector>
+#include <stack>
+#include <unordered_set>
+using namespace std;
+
+//dfs to store vertices in a stack as per their finish time
+void dfs(vector<int>* edges, int start, unordered_set<int> &visited, stack<int> &finishStack) {
+    visited.insert(start);
+    for (int i = 0; i < edges[start].size(); i++) {
+        int adjacent = edges[start][i];
+        if (visited.count(adjacent) == 0) {
+            dfs(edges, adjacent, visited, finishStack);
+        }
+    }
+    finishStack.push(start);
+}
+
+//dfs of a transpose graph
+void dfs2(vector<int>* edges, int start, unordered_set<int>* component, unordered_set<int> & visited) {
+    visited.insert(start);
+    component->insert(start);
+    for (int i = 0; i < edges[start].size(); i++) {
+        int adjacent = edges[start][i];
+        if (visited.count(adjacent) == 0) {
+            dfs2(edges, adjacent, component, visited);
+        }
+    }
+}
+
+unordered_set<unordered_set<int>*>* getSCC(vector<int>* edges, vector<int>* edgesT, int n) {
+    unordered_set<int> visited;
+    stack<int> finishedVertices;
+    for (int i = 0; i < n; i++) {
+        if (visited.count(i) == 0) {
+            dfs(edges, i, visited, finishedVertices);
+        }
+    }
+    unordered_set<unordered_set<int>*>* output = new unordered_set<unordered_set<int>*>();
+    visited.clear();
+    while (finishedVertices.size() != 0) {
+        int element = finishedVertices.top();
+        finishedVertices.pop();
+        if (visited.count(element) != 0) {
+            continue;
+        }
+        unordered_set<int>* component = new unordered_set<int>();
+        dfs2(edgesT, element, component, visited);
+        output->insert(component);
+    }
+    return output;
+}
+
+int main() {
+    int n;    //no.of vertices
+    cout<<"Enter the no. of vertices : "<<endl;
+    cin >> n;
+    vector<int>* edges = new vector<int>[n];    //using a adjacency list to implement
+    vector<int>* edgesT = new vector<int>[n];    //transpose of a graph
+    int m;    // no.of edges
+    cout<<"Enter the no. of edges : "<<endl;
+    cin >> m;
+
+    for (int i = 0; i < m; i++) {
+        int j, k;
+        cin >> j >> k;
+        edges[j - 1].push_back(k - 1);
+        edgesT[k - 1].push_back(j - 1);
+    }
+
+    unordered_set<unordered_set<int>*>* components = getSCC(edges, edgesT, n);
+    unordered_set<unordered_set<int>*>::iterator it = components->begin();
+
+    while (it != components->end()) {
+        unordered_set<int>* component = *it;
+        unordered_set<int>::iterator it2 = component->begin();
+        while (it2 != component->end()) {
+            cout <<*it2  + 1<< " ";
+            it2++;
+        }
+        cout << endl;
+        delete component;
+        it++;
+    }
+
+    delete components;
+    delete [] edges;
+    delete [] edgesT;
+}
+
+/*INPUTS
+Enter the no. of vertices : 6
+Enter the no. of edges  :7
+1 2
+2 3
+3 4
+4 1
+3 5
+5 6
+6 5
+
+OUTPUT
+6 5
+2 3 4 1 */