70. Climbing Stairs

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?

递推公式为f(n)=f(n-1)+f(n-2)

每个元素仅依赖于前两个元素。

class Solution {
public:
    int climbStairs(int n) {
        int result[3] = {0,1,2};
        if(n < 3){
            return result[n];
        }
        
        int f1 = 1;
        int f2 = 2;
        int fn = 0;
        
        for(int i = 3; i <= n; i++ ){
            fn = f1 + f2;
            f1 = f2;
            f2 = fn;
        }
        return fn;
    }
};