func_code_string: #include <bits/stdc++.h>   using namespace std;   int f[150000];   int find(int x) {     if (x == f[x])       return f[x];     else {       return f[x] = find(f[x]);     }   }   int a[150000];   int num[150000];   int main() {     ios::sync_with_stdio(false);     int n, m;     while (cin >> n >> m) {       memset(a, 0, sizeof(a));       memset(num, 0, sizeof(num));       for (int i = 1; i <= n; i++) f[i] = i;       for (int i = 0; i < m; i++) {         int x, y;         cin >> x >> y;         a[x]++;         a[y]++;         x = find(x), y = find(y);         if (x != y) f[x] = y;       }       for (int i = 1; i <= n; i++) {         find(i);       }       for (int i = 1; i <= n; i++) {         num[f[i]]++;       }       bool flag = true;       for (int i = 1; i <= n; i++) {         if (a[i] != num[f[i]] - 1) {           flag = false;           break;         }       }       if (flag)         cout <<  YES  << endl;       else         cout <<  NO  << endl;     }     return 0;   }   