You are climbing a stair case. It takes n steps to reach to the top.

Each time you can either climb 1 or 2 steps. In how many distinct ways can you climb to the top?

Note: Given n will be a positive integer. 

int climbStairs(int n) 
{
    /*递归实现更好理解，但性能存在问题*/
    if(n <= 0)
        return 0;
    
    if(1 == n)
        return 1;
    
    if(2 == n)
        return 2;
    
    int one_step_before = 2;
    int two_steps_before = 1;
    int all_ways = 0;
    
    for(int i=2; i<n; i++)
    {
        all_ways = one_step_before + two_steps_before;
        /*更改two_steps_before与one_step_before的值，计算下一个阶梯的ways*/
        two_steps_before = one_step_before;
        one_step_before = all_ways;
    }
    return all_ways;
    
}