P16249 - Task ordering

#include <iostream>
#include <vector>
#include <queue>
#include <algorithm>
using namespace std;

int posicio(string w, const vector<string>& s, int esq, int dre) {
  if (esq > dre) return -1;
  else {
    int i = (esq + dre)/2;
    if (w < s[i]) return posicio(w,s,esq,i-1);
    else if (w > s[i]) return posicio(w,s,i+1,dre);
    else return i;
  }

}

int index(const vector<string>& s, string w) {
    return (posicio(w, s, 0, s.size()-1));
}

void topological_sort(const vector<vector<int> >& g, vector<int>& pointers, const vector<string>& v1){
    int n = g.size();
    vector<int> end;
    vector<int> visited(n,false);
    priority_queue<int,vector<int>,greater<int> > q;
    for(int i = 0; i < n; ++i){
        if(pointers[i] == 0){
            q.push(i);
        }
    }
    if (q.empty()) cout << "NO VALID ORDERING" <<endl;
      else{
        while(!q.empty()){
            int u = q.top();
            q.pop();
            if(!visited[u]){
                end.push_back(u);
                visited[u] = true;
                for(int i = 0; i < g[u].size(); ++i){
                    --pointers[g[u][i]];
                    if (pointers[g[u][i]] == 0) q.push(g[u][i]);
                }
            }
        }
        if (end.size() == n){
            for(int i = 0; i < n; ++i){
                cout << v1[end[i]];
            }
            cout << endl;
        }
        else cout << "NO VALID ORDERING" <<endl;
    }
}

int main(){
    int n,m;
    while(cin >> n){
        vector<string> v1(n);
        for(int i = 0; i < n; ++i){
            cin >> v1[i];
        }
        sort(v1.begin(),v1.end());
        cin >> m;
        vector<vector<int> > g(n);
        vector<int> pointers(n,0);
        for(int i = 0; i < m; ++i){
            string s,h;
            cin >> s >> h;
            int u = index(v1,s);
            int v = index(v1,h);
            g[u].push_back(v);
            ++pointers[v];
        }
        topological_sort(g,pointers, v1);
    }

}