发现 阶乘太厉害了 ，这样搞很容易溢出，导致计算出错。
class Solution {
public:
int climbStairs(int n) {
int ret=0;
for(int i=0;i<=n/2;i++)
{
int nSteps=n;
nSteps=nSteps-i*2;
ret+=calculate(i,nSteps);
printf("the i=%d,the nStep=%d,the ret=%d\n",i,nSteps,ret);
}
return ret;
}
//permutation 排列 //combination 组合
int calculate(int a,int b)
{
return (int)(factorial(a+b)/(factorial(a)*factorial(b)));
}
//factorial 阶乘
unsigned long long factorial(unsigned long long n)
{
unsigned long long ret=1;
for(int i=1;i<=n;i++)
{
ret*=i;
}
return ret;
}
};
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