Climbing stairs [https://leetcode.com/problems/climbing-stairs]

climbing_stairs.js

/**
 * @param {number} n
 * @return {number}
 */
 var climbStairs = function(n) {
    if (n === 1) {
        return 1;
    }

    let first = 1;
    let second = 1;

    for (let i = n - 2; i >= 1; i--) {
        let temp = first + second;
        first = second;
        second = temp;
    }

    return first + second;
};