Leetcode 70: 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?
爬楼梯问题，其实是一个斐波那契数列问题，fn = fn-1+ fn-2； 使用dp的思想求解，时间复杂度O（n), 空间复杂度O（1）
class Solution {
public:
    int climbStairs(int n) {
        //sanity check
        if (n <= 1) return 1;
       // if (n == 2) return 2;
        int pre = 1;
        int prep =1;
        int cur = 0;
        for (int i = 2; i <= n; i++) {
            cur = pre + prep;
            prep = pre;
            pre =  cur;
        }
        return cur;
    }
};