func_code_string: #include <bits/stdc++.h>   using namespace std;   int main() {     static int f[10000];     int n, ok = false;     scanf( %d , &n);     for (register int i = 1; i <= n; ++i) scanf( %d , f + i);     for (int i = 1; i <= n; ++i)       if (f[f[f[i]]] == i) ok = true;     puts(ok ?  YES  :  NO );   }   