题目

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，一次可以爬1梯，或者2梯，那么如果楼梯有n梯，那么爬到n梯有多少种可能

• Example 1

Input: 2
Output: 2
Explanation: There are two ways to climb to the top.
1. 1 step + 1 step
2. 2 steps

• Example 2

Input: 3
Output: 3
Explanation: There are three ways to climb to the top.
1. 1 step + 1 step + 1 step
2. 1 step + 2 steps
3. 2 steps + 1 step

• 思路
  当我们到达n层，可以选择从n-1层上来，也可以选择从n-2层上来，类似于斐波那契函数:
  dp[n] = dp[n-1] + dp[n-2]

• 解法

var climbStairs = function(n) {
  if (n === 1) return 1;
  if (n === 2) return 2;

  // n等于2的时候，有2中方法【1，1】和【2】,到达梯子2层
   // n等于1的时候，有1中方法【1】到达梯子1层

  let oneStep = 2, 
  let twoSteps = 1, 
  let current;

  //到达这里，说明n>=3
  // n=3，可以从2层上来，也可以从1层上来
  for (let i = 2; i < n; i++) {
      current = oneStep + twoSteps;
      twoSteps = oneStep;
      oneStep = current;
  }
  return current;
};