Apply DFS for Topological Sorting of Directed Acyclic Graph in C++

Topological sorting of DAG (Directed Acyclic Graph) is a linear ordering of vertices such that for every directed edge uv, where vertex u comes before v in the ordering. If the graph is not a DAG, Topological Sorting for a graph is not possible.

Functions and pseudocodes

Begin
   function topologicalSort():
   a) Mark the current node as visited.
   b) Recur for all the vertices adjacent to this vertex.
   c) Push current vertex to stack which stores result.
End
Begin
   function topoSort() which uses recursive topological sort() function:
   a) Mark all the vertices which are not visited.
   b) Call the function topologicalSort().
   c) Print the content.
End

Example

#include<iostream>
#include <list>
#include <stack>
using namespace std;
class G {
   int n;
   list<int> *adj;
   //declaration of functions
   void topologicalSort(int v, bool visited[], stack<int> &Stack);
   public:
   G(int n); //constructor
   void addEd(int v, int w);
   void topoSort();
};
G::G(int n) {
   this->n = n;
   adj = new list<int> [n];
}
void G::addEd(int v, int w) // add the edges to the graph. {
   adj[v].push_back(w); //add w to v’s list
}
void G::topologicalSort(int v, bool visited[], stack<int> &Stack) {
   visited[v] = true; //mark current node as visited
   list<int>::iterator i;
   //Recur for all the vertices adjacent to this vertex.
   for (i = adj[v].begin(); i != adj[v].end(); ++i)
      if (!visited[*i])
         topologicalSort(*i, visited, Stack);
         Stack.push(v);
}
void G::topoSort() {
   stack<int> Stack;
   bool *visited = new bool[n];
   //Mark all the vertices which are not visited.
   for (int i = 0; i < n; i++)
      visited[i] = false;
      for (int i = 0; i < n; i++)
         if (visited[i] == false)
            //Call the function topologicalSort().
            topologicalSort(i, visited, Stack);
         while (Stack.empty() == false) {
            cout << Stack.top() << " "; //print the element
            Stack.pop();
         }
}
int main() {
   G g(6);
   g.addEd(4, 2);
   g.addEd(5, 1);
   g.addEd(4, 0);
   g.addEd(3, 1);
   g.addEd(1, 3);
   g.addEd(3, 2);
   cout << " Topological Sort of the given graph \n";
   g.topoSort();
   return 0;
}

Output

Topological Sort of the given graph
5 4 1 3 2 0