Climbing Stairs

Problem Description

You are climbing a staircase. It takes n steps to reach the top.

Each time you can either climb 1 or 2 steps. In how many distinct ways can you climb to the top?

Example 1:

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

Example 2:

Input: n = 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

Constraints:

• 1 <= n <= 45

Solution (JavaScript)

/**
 * @param {number} n
 * @return {number}
 */
var climbStairs = function(n) {
    // https://leetcode.com/problems/climbing-stairs/solutions/25299/basically-it-s-a-fibonacci/comments/230554
    if (n<=2){
        return n;
    }
    let twoStepBefore = 1; // when n = 1
    let oneStepBefore = 2; // when n = 2
    let curStep = 0;
    for(let i = 3; i <= n; i++){
        curStep = twoStepBefore + oneStepBefore;
        twoStepBefore = oneStepBefore;
        oneStepBefore = curStep;
    }
    return curStep;
};