Luogu 4492 [HAOI2018]苹果树 组合数

 1 #include<cstdio>
 2 #include<cstring>
 3 #include<algorithm>
 4 #include<cmath>
 5 #include<iostream>
 6 using namespace std;
 7 #define LL long long
 8 LL n;LL p;
 9 LL f[2010][2010]={};
10 LL zu[2010][2010]={};
11 LL t[2010]={};
12 int main(){
13     scanf("%lld%lld",&n,&p);
14     LL ans=0; zu[0][0]=1; t[0]=1;
15     for(LL i=1;i<=n;i++)t[i]=(t[i-1]*i)%p;
16     for(LL i=1;i<=n;i++){
17         zu[i][0]=1;
18         for(LL j=1;j<=i;j++)zu[i][j]=(zu[i-1][j]+zu[i-1][j-1])%p;
19     }
20     for(LL i=1;i<=n;i++){
21         f[i][0]=1;
22         for(int j=1;j<=n;j++){
23             f[i][j]=(f[i][j-1]*(i+j-1))%p;
24         }
25     }
26     for(LL i=2;i<=n;i++){
27         for(LL j=n-i+1;j>0;j--){
28             LL w=(zu[n-i][j-1]*t[j])%p;
29             w=(w*((t[i]*f[i-1][n-i-j+1])%p))%p;
30             w=(w*((j*(n-j))%p))%p;
31             ans=(ans+w)%p;
32         }
33     }
34     printf("%lld\n",ans);
35     return 0;
36 }