Climbing Stairs

题目来自于：

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?

思路：实际上就是一个斐波那契数列。f(i) = f(i-1) + f(i-2) ，第i阶可以是从第i-2阶爬上来的，也可以是从第i-1阶爬上来的。进一步发现在迭代到第i阶时，我们只要保存前面的f(i-1)和f(i-2），所以只要定义两个变量就可以，不用定义一个数组。

language:javascript

/** * @param {number} n * @return {number} */var climbStairs = function(n) {    var num1 = 1;    var num2 = 2;    var num3;    if(n == 1) return num1;    if(n == 2) return num2;    for(var i=2;i<n;i++){        num3 = num1 + num2;        num1 = num2;        num2 = num3;    }    return num3;};