func_code_string: #include <bits/stdc++.h>   using namespace std;   long long n, a[2000005], num[2000005], ans;   long long ksm(long long x, long long y) {     long long ans = 1, t = x;     while (y) {       if (y & 1) ans = ans * t % 1000000007;       t = t * t % 1000000007;       y >>= 1;     }     return ans;   }   int main() {     scanf( %lld , &n);     for (long long i = 1; i <= n; i++) {       scanf( %lld , &a[i]);       if (a[i] != -1) num[a[i]]++;     }     for (long long i = 1; i <= n; i++) {       if (a[i] == -1) ans = (ans + ksm(2, num[i]) - 1 + 1000000007) % 1000000007;     }     printf( %lld , (ksm(2, n - 1) - 1 - ans + 1000000007) % 1000000007);   }   