3月3日(6) Climbing Stairs

class Solution {
public:
    int climbStairs(int n) {
        if (n==0) return 0;
        
        int n1 = 0;
        int n2 = 1;
        
        for (int i=0; i<n; ++i)
        {
            int t = n2;
            n2 = n1 + n2;
            n1 = t; 
        }
        return n2;
    }
};