func_code_string: #include <bits/stdc++.h>   using namespace std;   vector<int> g1[30000 + 100], g2[30000 + 100];   set<int> st;   int Next[30000 + 100], Prev[30000 + 100];   int main() {     ios_base::sync_with_stdio(false);     cin.tie(NULL);     int n, m, start;     cin >> n >> m;     for (int i = 1; i <= m; i++) {       int u, v;       cin >> u >> v;       g1[u].push_back(v);       g2[v].push_back(u);     }     for (int i = 1; i <= n; i++) {       sort(g1[i].begin(), g1[i].end());       sort(g2[i].begin(), g2[i].end());     }     start = 0;     st.insert(0);     st.insert(1);     Next[start] = 1;     Prev[1] = 0;     for (int i = 2; i <= n; i++) {       vector<int> d;       bool flag = false;       for (auto it : g1[i]) {         if (it > i) continue;         int t1 = Next[it];         if (!binary_search(g1[i].begin(), g1[i].end(), t1)) {           flag = true;           Next[it] = i;           Prev[t1] = i;           Next[i] = t1;           Prev[i] = it;           break;         } else {           st.erase(it);           d.push_back(it);         }       }       if (!flag) {         for (auto it : g2[i]) {           if (it > i) continue;           int t1 = Prev[it];           if (!binary_search(g1[t1].begin(), g1[t1].end(), i)) {             flag = true;             Next[t1] = i;             Prev[it] = i;             Next[i] = it;             Prev[i] = t1;             break;           } else {             st.erase(t1);             d.push_back(t1);           }         }       }       if (!flag) {         auto it = st.begin();         int t1 = Next[*it];         Next[*it] = i;         Prev[t1] = i;         Next[i] = t1;         Prev[i] = *it;       }       for (auto it : d) {         st.insert(it);       }       st.insert(i);     }     int now = Next[0];     while (now) {       cout << now <<    ;       now = Next[now];     }   }   