func_code_string: #include <bits/stdc++.h>   using namespace std;   int i, j, k, l, s[1000005], n, m, tot, last[1000005], Next[1000005],       to[1000005], dfn[1000005], low[1000005];   int vis[1000005], q[1000005], Q[1000005], q1[1000005], r, r1, G[1000005], T;   vector<int> gt;   int ans[1000005], A[1000005], B[1000005], a[1000005], b[1000005], x, y;   inline void add(int x, int y) {     Next[++tot] = last[x];     last[x] = tot;     to[tot] = y;   }   inline void tarjan(int x) {     dfn[x] = low[x] = ++tot;     q[++r] = x;     vis[x] = 1;     for (int i = last[x]; i; i = Next[i]) {       if (!dfn[to[i]])         tarjan(to[i]), low[x] = min(low[x], low[to[i]]);       else if (vis[to[i]])         low[x] = min(low[x], low[to[i]]);     }     if (low[x] == dfn[x]) {       ++T;       while (q[r] != x) {         G[q[r]] = T;         vis[q[r]] = 0;         r--;         s[T]++;       }       G[q[r]] = T;       vis[q[r]] = 0;       r--;       s[T]++;     }   }   inline void bfs(int x) {     int l = 0, r = 1;     Q[1] = x;     while (l < r) {       int k = Q[++l];       for (int i = last[k]; i; i = Next[i]) {         if (!vis[to[i]]) vis[Q[++r] = to[i]] = 1;       }     }   }   inline bool check(int x, int y) {     int s = 0;     int p = x;     while (x) {       s += x & 1;       x >>= 1;     }     x = y;     while (x) {       s -= x & 1;       x >>= 1;     }     if (s > 0) return true;     if (s < 0) return false;     if (s == 0 && p != (1 << r1) - 1) return true;     return false;   }   int main() {     scanf( %d%d , &n, &m);     if (n == 1) return puts( YES ), 0;     for (i = 1; i <= m; i++) scanf( %d%d , &x, &y), add(x, y);     tot = 0;     for (i = 1; i <= n; i++)       if (!dfn[i]) tarjan(i);     for (i = 1; i <= n; i++)       for (j = last[i]; j; j = Next[j]) {         B[G[to[j]]]++, A[G[i]]++;         if (G[to[j]] != G[i]) a[G[i]] = b[G[to[j]]] = 1;       }     for (i = 1; i <= T; i++)       if (!A[i] && !B[i]) return puts( NO ), 0;     for (i = 1; i <= n; i++) {       if (!A[G[i]])         gt.push_back(i);       else if (!B[G[i]])         q1[++r1] = i;     }     for (i = 1; i <= T; i++) {       if (A[i] && B[i] && r1 && (!a[i] || !b[i])) return puts( NO ), 0;     }     if (!r1 && T > 1) return puts( NO ), 0;     for (i = 1; i <= r1; i++) {       for (j = 1; j <= n; j++) vis[j] = 0;       bfs(q1[i]);       for (j = 0; j < gt.size(); j++)         if (vis[gt[j]]) ans[i] += (1 << (j));     }     for (i = 1; i < (1 << r1); i++) {       int x = i;       int s = 0, t = 1;       while (x) {         if (x & 1) s |= ans[t];         t++;         x >>= 1;       }       if (check(i, s)) return puts( NO ), 0;     }     puts( YES );   }   