师范大学 e: skyscrapers

#include<iostream>
#include<stdio.h>
#include<cstring>
#include<cstdlib>
using namespace std;
typedef __int64 LL;
const LL mod = 1000000007;
LL cnm[5002][5002];
LL  dp[5002][5002];

void prepare()
{
LL i,j;
for(i=0;i<=5000;i++)
{
cnm[0][i]=1;
cnm[i][0]=1;
}
for(i=1;i<=5000;i++)
{
for(j=1;j<=i&&j<=5000;j++)
{
if(i==j) { cnm[i][j]=1;continue;}
if(j==1) { cnm[i][j]=i;continue;}
cnm[i][j]=(cnm[i-1][j]+cnm[i-1][j-1])%mod;
}
}
dp[0][0]=1;
for(i=1;i<=5000;i++)
for(j=1;j<=5000;j++)
dp[i][j]=(dp[i-1][j-1]+((i-1)*dp[i-1][j])%mod)%mod;
}
int main()
{
LL n,a,b,cur;
LL l,r;
prepare();
while(scanf("%I64d%I64d%I64d",&n,&a,&b)>0)
{
if(n==0&&a==0&&b==0)break;
l=a;r=n-b+1;
for(cur=0;l<=r;l++)
{
cur=( cur+((cnm[n-1][l-1]*dp[l-1][a-1])%mod)*dp[n-l][b-1])%mod;
}
printf("%I64d\n",cur);
}
return 0;
}