poj942Paths on a Grid

http://poj.org/problem?id=1942

建立数学模型 C(a,b);

此题就是考察排列组合 就是数据卡的紧 wa了两次

#include<iostream>
using namespace std;
int main()
{
 unsigned long long int  n,m,s,a,b;
 unsigned long long int i,j;
    while(cin>>n>>m)
    {
        if(n==0&&m==0) break;
        if(n<m)
        {
             unsigned long long int t=n;
            n=m;
            m=t;
        }
        s=1;a=n+m;b=m;
        for(i=a-b+1,j=1;i<=a,j<=b;i++,j++)
        {
            s=s*i/j;
        }
        cout<<s<<endl;

    }
    return 0;
}