2013 Multi-University Training Contest 1 Partition

这题主要是推公式……

#include<iostream>
#include<stdio.h>
#include<algorithm>
#include<iomanip>
#include<cmath>
#include<string>
#include<vector>
#define ll __int64
#define pi acos(-1.0)
using namespace std;
ll mod=1000000007;
ll pows(ll a,ll b)
{
    ll ans=1;
    while(b)
    {
        if(b&1) ans=(ans*a)%mod;
        b>>=1;
        a=(a*a)%mod;
    }
    return ans;
}
int main()
{
    int t;
    ll ans,n,k,m;
    cin>>t;
    while(t--)
    {
        scanf("%I64d%I64d",&n,&k);
        m=n-k+1;
        if(n<k) ans=0;
        else if(m==1) ans=1;
        else if(m==2) ans=2;
        else
        {
            ans=(m+2)*pows(2,m-3)%mod;
            if(ans<0) ans=(ans%mod+mod)%mod;
        }
        printf("%I64d\n",ans);
    }
    return 0;
}