leetCode 70. Climbing Stairs

题目如下：

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?
Note: Given n will be a positive integer.

思路：

当n＝1，结果为1

当n＝2，结果为2

当n＝3，结果为3

当n＝4，结果为7

易递推[n]=[n-1]+[n-2]，是一个斐波纳切数列。

本题代码：

int climbStairs(int n) {
    if (n<=0) return 0;
    if (n==1) return 1;
    if (n==2) return 2;
    
    int one_step_before = 2;
    int two_steps_before = 1;
    int all_ways = 0;
    
    for(int i=3; i<=n; i++) {
        all_ways = one_step_before + two_steps_before;
        two_steps_before = one_step_before;
        one_step_before = all_ways;
    }
    return all_ways;
}