Description

In how many ways can you choose k elements out of n elements, not taking order into account? 
Write a program to compute this number.

Input

The input will contain one or more test cases. 
Each test case consists of one line containing two integers n (n>=1) and k (0<=k<=n). 
Input is terminated by two zeroes for n and k.

Output

For each test case, print one line containing the required number. This number will always fit into an integer, i.e. it will be less than 2 31
Warning: Don't underestimate the problem. The result will fit into an integer - but if all intermediate results arising during the computation will also fit into an integer depends on your algorithm. The test cases will go to the limit. 

Sample Input

4 2
10 5
49 6
0 0

Sample Output

6
252
13983816
解题思路:简单求组合数,数据比较小,直接暴力枚举运算即可!
AC代码:
 #include<iostream>
using namespace std;
int main(){
int n,k;long long ans;//开long long,避免数据溢出
while(cin>>n>>k&&(n+k)){
if(n-k<k)k=n-k;
ans=;
for(int i=;i<=k;++i)ans=ans*(n-i+)/i;
cout<<ans<<endl;
}
return ;
}

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