hannnnah_j’s Biological Test HDU - 5894

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挡板法求解​

 

#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define M 1000000007

ll quickpow(ll a,ll b)
{
    ll ans;
    ans=1;
    while(b>0)
    {
        if(b%2==1)
        {
            ans=(ans*a)%M;
        }
        a=(a*a)%M;
        b/=2;
    }
    return ans;
}

ll getc(ll n,ll k)
{
    ll ans,sum1,sum2,i,j;
    sum1=1,sum2=1,i=n,j=1;
    while(k--)
    {
        sum1=(sum1*i)%M;
        sum2=(sum2*j)%M;
        i--,j++;
    }
    ans=(sum1*quickpow(sum2,M-2))%M;
    return ans;
}

int main()
{
    ll ans;
    int t,n,m,k;
    scanf("%d",&t);
    while(t--)
    {
        scanf("%d%d%d",&n,&m,&k);
        if(m==1)
        {
            printf("%d\n",n);
            continue;
        }
        if(n<m+m*k)
        {
        printf("0\n");
        continue;
    }
        ans=(((getc(n-m*k-1,m-1)*n)%M)*(quickpow(m,M-2)%M))%M;
        printf("%lld\n",ans);
    }
    return 0;
}