Number of stair climbing methods of leetcode

70. climb stairs
    Suppose you're climbing the stairs . need n You can reach the top of the building .

Every time you climb 1 or 2 A stair . How many different ways can you climb to the top of the building ？

Example 1：

Input ：n = 2
Output ：2
explain ： There are two ways to climb to the top .

1. 1 rank + 1 rank
2. 2 rank
   Example 2：

Input ：n = 3
Output ：3
explain ： There are three ways to climb to the top .

1. 1 rank + 1 rank + 1 rank
2. 1 rank + 2 rank
3. 2 rank + 1 rank

Tips ：

1 <= n <= 45

var climbStairs = function(n) {
    
  if (n === 1) return 1
  if (n === 2) return 2
  let n_1 = 2, n_2 = 1
  let cur = 0
  for (let i = 3; i <= n; i++) {
    
    cur = n_1 + n_2
    n_2 = n_1
    n_1 = cur
  }
  return cur
}

res = climbStairs(3)

console.log(res);