poj 2249 Binomial Showdown

题目链接：http://poj.org/problem?id=2249

 

题目描述：

 

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

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题目分析：

组合数，注意要*一个数/一个数同时进行，不然中间可能某一步会越界

还有就是最后输出的时候要考虑不确定度

 

AC代码：

 

#include<iostream>
#include<cstdio>
#include<cstdlib>
#include<cmath>
#include<cstring>
#include<algorithm>
using namespace std;
int a,b;
int main()
{
    while(1)
    {
        cin>>a>>b;
        if(a==0&&b==0)
            break;
        b=min(b,a-b);
        double x=1.0;
        while(b>0)
        {
            x*=(a*1.0/b);
            a--;
            b--;
        }
        printf("%d\n",(int)(x+0.005));
    }
    return 0;
}