func_code_string: #include <bits/stdc++.h>   using namespace std;   int a[2010];   long long C[2010][2010];   long long A[2010];   bool use[2010];   int n, m, N;   void init() {     memset(use, 0, sizeof(use));     C[0][0] = 1;     for (int i = 1; i <= 2000; ++i) C[i][0] = 1;     for (int i = 1; i <= 2000; ++i)       for (int j = 1; j <= i; ++j)         C[i][j] = (C[i - 1][j - 1] + C[i - 1][j]) % 1000000007;     A[0] = 1;     A[1] = 1;     for (int i = 2; i <= 2000; ++i) A[i] = (A[i - 1] * i) % 1000000007;     n = 0;   }   void solve() {     long long Ans = 1;     Ans = A[n];     for (int i = 1; i <= N; ++i)       if (a[i] == -1 && !use[i]) m++;     long long tmp = 0;     for (int i = 1; i <= m; ++i) {       if (i & 1)         tmp = (tmp + (C[m][i] * A[n - i]) % 1000000007) % 1000000007;       else         tmp = (1000000007 + tmp - (C[m][i] * A[n - i]) % 1000000007) % 1000000007;     }     cout << (Ans + 1000000007 - tmp) % 1000000007;   }   int main() {     scanf( %d , &N);     init();     for (int i = 1; i <= N; ++i) {       scanf( %d , &a[i]);       if (a[i] != -1)         use[a[i]] = 1;       else         n++;     }     solve();     return 0;   }   