Topological sort implementation using C++ program

Algorithm:

To solve this problem we follow this process:

1. We take two stacks and a set. We use the set to mark the nodes.
2. We start with any of the vertexes and push it into the stack and mark it (insert into the set).
3. We take the top element and check for its unvisited adjacent vertices if it exists then push it into the stack and if not then all the nodes are visited therefore we pop that node from the stack and place it into the other stack.
4. We repeat step 3 until we visited all the nodes.

C++ implementation:

#include <bits/stdc++.h>
using namespace std;

void addedge(list<int>* ls, int x, int y)
{
    ls[x].push_back(y);
}

string topological_sort(list<int>* ls, int k)
{
    char arr[k];
    stack<int> s;
    set<int> st;
    int ind = k - 1;
    for (int i = k - 1; i >= 0; i--) {
        if (st.find(i) == st.end()) {
            s.push(i);
            st.insert(i);
            //check all the non visited nodes
            while (!s.empty()) {
                int p = s.top();
                list<int>::iterator it;
                int temp = 0;
                //check its adjacent non visited nodes
                for (it = ls[p].begin(); it != ls[p].end(); it++) {
                    if (st.find(*it) == st.end()) {
                        st.insert(*it);
                        s.push(*it);
                        temp = 1;
                    }
                }
                //if all adjaceny nodes are visited then pop that element from stack
                if (temp == 0) {
                    char ch = (char)(p + 'A');
                    arr[ind] = ch;
                    ind--;
                    s.pop();
                }
            }
        }
    }
    return arr;
}
int main()
{
    int k = 7;
    list<int> ls[k];
    addedge(ls, 0, 2);
    addedge(ls, 0, 3);
    addedge(ls, 1, 2);
    addedge(ls, 1, 4);
    addedge(ls, 4, 5);
    addedge(ls, 3, 5);
    addedge(ls, 5, 6);
    cout << topological_sort(ls, 7) << endl;
    return 0;
}

Output

ABCDEFG