Merge Sorted Array

Problem statement

You are given two integer arrays nums1 and nums2, sorted in non-decreasing order, and two integers m and n, representing the number of elements in nums1 and nums2 respectively.

Merge nums1 and nums2 into a single array sorted in non-decreasing order.

The final sorted array should not be returned by the function, but instead be stored inside the array nums1. To accommodate this, nums1 has a length of m + n, where the first m elements denote the elements that should be merged, and the last n elements are set to 0 and should be ignored. nums2 has a length of n.

Example 1:

Input: nums1 = [1,2,3,0,0,0], m = 3, nums2 = [2,5,6], n = 3Output: [1,2,2,3,5,6]Explanation: The arrays we are merging are [1,2,3] and [2,5,6].The result of the merge is [1,2,2,3,5,6] with the underlined elements coming from nums1.

Example 2:

Input: nums1 = [1], m = 1, nums2 = [], n = 0Output: [1]Explanation: The arrays we are merging are [1] and [].The result of the merge is [1].

Example 3:

Input: nums1 = [0], m = 0, nums2 = [1], n = 1Output: [1]Explanation: The arrays we are merging are [] and [1].The result of the merge is [1].Note that because m = 0, there are no elements in nums1. The 0 is only there to ensure the merge result can fit in nums1.

Constraints:

• nums1.length == m + n
• nums2.length == n
• 0 <= m, n <= 200
• 1 <= m + n <= 200
• -109 <= nums1[i], nums2[j] <= 109

Follow up: Can you come up with an algorithm that runs in O(m + n) time?

My solution

/**
 * @param {number[]} nums1
 * @param {number} m
 * @param {number[]} nums2
 * @param {number} n
 * @return {void} Do not return anything, modify nums1 in-place instead.
 */
var merge = function(nums1, m, nums2, n) {
    let index = nums1.length - 1;
    let indexM = m - 1;
    let indexN = n - 1;
    
    while (index >= 0) {
        if (indexM < 0) {
            nums1[index] = nums2[indexN--]
        } else if(indexN < 0) {
            nums1[index] = nums1[indexM--]
        } else {
            if (nums1[indexM] > nums2[indexN]) {
                nums1[index] = nums1[indexM--]
            } else {
                nums1[index] = nums2[indexN--]
            }
        }
        
        index--;
    }
};