poj 3761 Bubble Sort_快速幂

题意：问你冒泡排序第i次排序，一共排了多少次

套公式K!((K + 1) ^ (N - K) - K ^ (N - K))

#include <iostream>
#include<cstdio>
#include<cstring>
using namespace std;
#define LL long long
#define N 1000010
#define M 20100713
LL a[N];
int _pow(LL v,int k){
	LL res=1;
	while(k){
		if(k&1){
			res=res*v;
			res%=M;
		}
		v=v*v;
		v%=M;
		k>>=1;
	}
	return res;
}
int main(int argc, char** argv) {
	int t,i,n,k;
	a[0]=a[1]=1;
	for(i=2;i<N;i++)
		a[i]=a[i-1]*i%M;
	scanf("%d",&t);
	while(t--){
		LL sum=0;
		scanf("%d%d",&n,&k);
		sum=(_pow(k+1,n-k)-_pow(k,n-k))%M;
		if(sum<0)
			sum+=M;
		printf("%lld\n",sum*a[k]%M);
	}
	return 0;
}