class Solution {
public:
    int climbStairs(int n) {
        if(n == 1) return 1;
        if(n == 2) return 2;
        
           int pre = 1;
        int current = 1;
        for(int i = 2; i <= n; i++){
            int tmp = current+pre;
            pre = current;
            current = tmp;
        }
        return current;     
    }
    
    int climbStairsRecursive(int n){
         if(n == 1) return 1;
        if(n == 2) return 2;
        return climbStairsRecursive(n-1)+climbStairsRecursive(n-2);
    }
};