[Leetcode 29] 70 Climbing Stairs

Probelm:

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?

Analysis:

Simplest DP problem, 

Sn = Sn-1 + Sn-2 with S1 = 1 and S2 = 2;

Code:

class Solution {
public:
    int climbStairs(int n) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        if (n == 1) return 1;
        if (n == 2) return 2;
        
        int s1=1, s2=2, s3;
        
        for (int i=2; i<n; i++) {
            s3 = s1 + s2;
            s1 = s2;
            s2 = s3;
        }
        
        return s3;
    }
};

Attention: