Climbing Stairs 爬楼梯问题，每次可以走1或2步，爬上n层楼梯总方法 （变相fibonacci）

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?

当n=1时，有1种方法，即直接走1步

当n=2时，有2方法：连续走2步，或直接走两步

对于n，设f(n)为总方法，则 f(n) = f(n-1)+f(n-2)  

ps:f(n-1)即第一次走一步的走法，

    f(n-2)即第一次走两步的走法

归回fibnacci问题解法：

class Solution {
public:
    int climbStairs(int n) {
        if(n < 0)
            return -1;
        int res[] = {0,1};
        if(n<2)
            return res[n];
            
        int fib1 = 0;
        int fib2 = 1;
        
        int result = 1;
        
        for(int i = 1 ; i <= n ; i++){
            result = fib1 + fib2;
            fib1 = fib2;
            fib2 = result;
        }
        
        return result;
    }
};