POJ 2249

简单求组合数，直接运用公式。注意C(n,k)=C(n,n-k)。

#include <iostream>
#include <cstdio>
#include <algorithm>
#define LL __int64
using namespace std;

LL myc(LL n, LL r){
	LL sum=1;
	for(LL i=1;i<=r;i++)
	sum=sum*(n+1-i)/i;
	return sum;
}

int main(){
	LL n,r;
	while(scanf("%I64d%I64d",&n,&r),n||r){
		r=r>n-r?n-r:r;
		LL res=myc(n,r);
		printf("%I64d
",res);
	}
	return 0;
}