传送门：https://www.lydsy.com/JudgeOnline/problem.php?id=5305

【题解】

 1 # include <bits/stdc++.h>
 2 using namespace std;
 3 
 4 const int M = 2010;
 5 
 6 int n, mod;
 7 int C[M][M], fac[M];
 8 
 9 int main() {
10     cin >> n >> mod;
11     fac[0] = 1;
12     for (int i=1; i<=n; ++i) fac[i] = 1ll * fac[i-1] * i % mod; 
13     C[0][0] = 1;
14     for (int i=1; i<=n; ++i) {
15         C[i][0] = 1;
16         for (int j=1; j<=i; ++j) {
17             C[i][j] = C[i-1][j] + C[i-1][j-1];
18             if(C[i][j] >= mod) C[i][j] -= mod;
19         }
20     }
21     int ans = 0;
22     for (int i=2; i<=n; ++i)
23         for (int sz=1; sz<=n-i+1; ++sz) {
24             int tot = 1ll * fac[n-sz-1] * i % mod * (i-1) % mod * fac[sz] % mod * C[n-i][sz-1] % mod;
25             ans += 1ll * tot * sz % mod * (n-sz) % mod;
26             if(ans >= mod) ans -= mod;
27         }
28         
29     cout << ans % mod << endl;    
30     return 0;
31 }

View Code