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?

递归模式很容易想到，但是超时啦。

class Solution {public:    int climbStairs(int n) {        if(n<=2){            return n;        }else{            return climbStairs(n-1)+climbStairs(n-2);        }    }};

所以采用非递归方式

class Solution {public:    int climbStairs(int n) {        if(n<=2){            return n;        }else{            int pre1 = 1;            int pre2 = 2;                        for(int i=3;i<=n;i++){                int temp = pre1 + pre2;                pre1 = pre2;                pre2 = temp;            }                        return pre2;        }    }};