Topological sort using DFS without recursion

Question

I know the common way to do a topological sort is using DFS with recursion. But how would you do it using stack instead of recursion? I need to obtai

Answer 1

The Graph structure is as folows

N: number of vertices adj[]: input graph

vector<int> topoSort(int V, vector<int> adj[]) {
    stack<int> s;
    vector<int> f(V,0);
    
    stack<int> out; 
    int i,j,x;
    for(i=0;i<V;i++){
        if(f[i]==0){
            s.push(i);
            
            while(!s.empty()){
                x = s.top();
                s.pop();
                if(f[x]==1){
                    out.push(x);
                    continue;
                }
                f[x] = 1;
                s.push(x);
                for(j=0;j<adj[x].size();j++){
                    if(f[adj[x][j]]==0){
                        s.push(adj[x][j]);
                    }
                }
            }
        }
    }
    
    vector<int> output;

    while(!out.empty()){
        x=out.top();
        out.pop();
        //cout << x << " ";
        output.push_back(x);
    }
    //cout << endl;
    
    return output;
}