POJ 2249 Binomial Showdown 求组合数

Binomial Showdown

4 2
10 5
49 6
0 0

6
252
13983816

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 231. 
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

Source

​​Ulm Local 1997​​

模板题，求组合数汇总（​​点这里​​）

#include<cstdio>  
#include<cstring>  
#include<cstdlib>  
#include<cctype>  
#include<cmath>  
#include<iostream>  
#include<sstream>  
#include<iterator>  
#include<algorithm>  
#include<string>  
#include<vector>  
#include<set>  
#include<map>  
#include<stack>  
#include<deque>  
#include<queue>
using namespace std;
typedef long long ll;
ll a[100000000];
 
ll C(ll n,ll m)
{

    if (n - m < m)
      m = n - m;
    a[0] = 1;
    for (int i = 1; i <= m; i++) 
    {
      a[i] = (n - i + 1) * a[i - 1] / i;
    }
  return a[m];
}
int main()
{
  ll n,m;
  while (~scanf("%lld%lld",&n,&m) && (n || m))
  {
    
    cout<<C(n,m)<<endl;
  } 
}