Given two sorted arrays, merge them into a new array that is also sorted.

merge-sorted-arrays.js

/**
 * Merge two sorted integer arrays nums1 and nums2 into nums1, sorted in non-decreasing order.
 * The time complexity of this function is O(m + n),
 * @param {number[]} nums1 The first sorted array containing elements to be merged.
 * @param {number} m The number of elements in nums1 (excluding additional space for merging).
 * @param {number[]} nums2 The second sorted array containing elements to be merged into nums1.
 * @param {number} n The number of elements in nums2.
 * @see https://leetcode.com/problems/merge-sorted-array/
 */
var merge = function(nums1, m, nums2, n) {
    // Initialize pointers for nums1, nums2, and the merged array
    let i = m - 1; // Pointer for the end of nums1
    let j = n - 1; // Pointer for the end of nums2
    let k = m + n - 1; // Pointer for the end of the merged array
    
    // Iterate until all elements from nums2 are merged into nums1
    while (j >= 0) {
        // If there are elements remaining in nums1 and nums1's element is greater than nums2's
        if (i >= 0 && nums1[i] > nums2[j]) {
            // Move the larger element from nums1 to the end of the merged array
            nums1[k--] = nums1[i--];
        } else {
            // Move the element from nums2 to the end of the merged array
            nums1[k--] = nums2[j--];
        }
    }
};

function mergeArraysES6(arrayA, arrayB) {
  let result = [...arrayA, ...arrayB];
  return result.sort((valueA, valueB) => valueA - valueB);
}

function mergeArrays(arrayA, arrayB) {
  var lengthA = arrayA.length;
  var lengthB = arrayB.length;
  var length = lengthA + lengthB;
  var result = []; // or new Array(length);
  var index = 0;
  var indexA = 0;
  var indexB = 0;

  while (length--) {
    var valueA = arrayA[indexA];
    var valueB = arrayB[indexB];

    if ((valueA < valueB || indexB == lengthB) && indexA < lengthA) {
      result[index++] = valueA;
      indexA++;
    } else {
      result[index++] = valueB;
      indexB++;
    }
  }

  return result;
}

var a = [1,2,4,5,7,334,1001];
var b = [-999, 0, 1, 33, 445, 1999];

console.log(mergeArraysES6(a, b));
console.log(mergeArrays(a, b));