Merge two sorted arrays. https://leetcode.com/problems/merge-sorted-array

merge1.js

const merge = (nums1, m, nums2, n) => {
  let index = m + n - 1; // Pointer to the current index being written to.
  let i = m - 1; // Pointer to value to compare in nums1.
  let j = n - 1; // Pointer to value to compare in nums2.
  
  // Move from right to left, compare values i, j and write larger at index.
  // If no more vaues in nums2, just choose nums1.
  while (index >= 0) {
    nums1[index--] = j < 0 || nums1[i] > nums2[j] ? nums1[i--] : nums2[j--];
  }
  
  return nums1;
};

merge2.js

const merge = (nums1, m, nums2, n) => {
    let i = m-1;
    let j = n-1;
    let index = m+n-1;

    while (j >= 0) {
        if (i >= 0 && nums1[i] > nums2[j]) {
            nums1[index--] = nums1[i--];
        }
        else {
            nums1[index--] = nums2[j--];
        }
    }

    return nums1;
};

znote.txt

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 = 3
Output: [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 = 0
Output: [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 = 1
Output: [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.