N - Binomial Showdown （组合数学）

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 ;
 }