https://leetcode.com/problems/climbing-stairs/
class Solution { public: int climbStairs(int n) { if (n==1) return 1; if (n==2) return 2; vector <int> stair; stair.push_back(1); stair.push_back(2); for (int i=2; i<n;i++) { stair.push_back(stair[i-1]+stair[i-2]); } return stair[n-1]; } };