func_code_string: #include <bits/stdc++.h>   using namespace std;   const int N = 2005;   const int M = 101;   const double EPS = 1e-9;   const int INF = 1e9;   long long gcd(long long a, long long b) { return b == 0 ? a : gcd(b, a % b); }   long long lcm(long long a, long long b) { return a * b / gcd(a, b); }   double getDist(complex<double> a, complex<double> b) {     return sqrt((a.real() - b.real()) * (a.real() - b.real()) +                 (a.imag() - b.imag()) * (a.imag() - b.imag()));   }   int dcmp(double x, double y) { return fabs(x - y) <= EPS ? 0 : x < y ? -1 : 1; }   long long mod = 1000000007, fib[N], dp[N][N];   long long power(long long base, int p) {     if (!p) return 1;     long long ret = power(base, p / 2);     ret *= ret;     ret %= mod;     if (p & 1) ret *= base, ret %= mod;     return ret;   }   long long modInverse(int a) { return power(a, mod - 2); }   long long ncr(int n, int r) {     return (((fib[n] * modInverse(fib[r])) % mod) * modInverse(fib[n - r])) % mod;   }   bool emty[N], taken[N];   long long solve(int n, int k) {     if (k == 0) return fib[n];     if (k == 1) return (fib[n] * n) % mod;     if (dp[n][k] != -1) return dp[n][k];     long long ret =         (solve(n, k - 1) * n) % mod + (solve(n + 1, k - 2) * (k - 1)) % mod;     return dp[n][k] = ret % mod;   }   int main() {     fib[0] = fib[1] = 1;     for (int i = 2; i < N; i++) fib[i] = (fib[i - 1] * i) % mod;     int n, free = 0, k = 0;     scanf( %d , &n);     for (int i = 1; i <= n; i++) {       int a;       scanf( %d , &a);       if (a == -1)         emty[i] = 1;       else         taken[a] = 1;     }     for (int i = 1; i <= n; i++) {       if (emty[i] == 1) {         if (taken[i] == 1)           free++;         else           k++;       }     }     memset(dp, -1, sizeof(dp));     cout << solve(free, k);     return 0;   }   