topological-sort.cpp

#include<iostream> 
#include <list> 
#include <stack> 
using namespace std; 
  
// Class to represent a graph 
class Graph 
{ 
    int V;    // No. of vertices' 
  
    // Pointer to an array containing adjacency listsList 
    list<int> *adj; 
  
    // A function used by topologicalSort 
    void topologicalSortUtil(int v, bool visited[], stack<int> &Stack); 
public: 
    Graph(int V);   // Constructor 
  
     // function to add an edge to graph 
    void addEdge(int v, int w); 
  
    // prints a Topological Sort of the complete graph 
    void topologicalSort(); 
}; 
  
Graph::Graph(int V) 
{ 
    this->V = V; 
    adj = new list<int>[V]; 
} 
  
void Graph::addEdge(int v, int w) 
{ 
    adj[v].push_back(w); // Add w to v’s list. 
} 
  
// A recursive function used by topologicalSort 
void Graph::topologicalSortUtil(int v, bool visited[],  
                                stack<int> &Stack) 
{ 
    // Mark the current node as visited. 
    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]) 
            topologicalSortUtil(*i, visited, Stack); 
  
    // Push current vertex to stack which stores result 
    Stack.push(v); 
} 
  
// The function to do Topological Sort. It uses recursive  
// topologicalSortUtil() 
void Graph::topologicalSort() 
{ 
    stack<int> Stack; 
  
    // Mark all the vertices as not visited 
    bool *visited = new bool[V]; 
    for (int i = 0; i < V; i++) 
        visited[i] = false; 
  
    // Call the recursive helper function to store Topological 
    // Sort starting from all vertices one by one 
    for (int i = 0; i < V; i++) 
      if (visited[i] == false) 
        topologicalSortUtil(i, visited, Stack); 
  
    // Print contents of stack 
    while (Stack.empty() == false) 
    { 
        cout << Stack.top() << " "; 
        Stack.pop(); 
    } 
} 
  
// Driver program to test above functions 
int main() 
{ 
    // Create a graph given in the above diagram 
    Graph g(6); 
    g.addEdge(5, 2); 
    g.addEdge(5, 0); 
    g.addEdge(4, 0); 
    g.addEdge(4, 1); 
    g.addEdge(2, 3); 
    g.addEdge(3, 1); 
  
    cout << "Following is a Topological Sort of the given graph \n"; 
    g.topologicalSort(); 
  
    return 0; 
}