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8. Merge Sorted Array

easyAsked at Chegg

Merge two sorted arrays in place into the first — Chegg uses this to test reverse-pointer thinking on tutor-rating roll-ups.

By Alex Chen, Founder, InterviewChamp.AI · Last verified

Problem

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 nums2 into nums1 as one sorted array in place.

Constraints

  • nums1.length == m + n
  • 0 <= m, n <= 200
  • Sorted non-decreasing

Examples

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]

Example 2

Input
nums1 = [1], m = 1, nums2 = [], n = 0
Output
[1]

Approaches

1. Concat + sort

Copy nums2 into nums1's tail and sort.

Time
O((m+n) log(m+n))
Space
O(1)
for (let i = 0; i < n; i++) nums1[m + i] = nums2[i];
nums1.sort((a, b) => a - b);

Tradeoff:

2. Reverse three-pointer

Fill from the back so we never overwrite unmerged nums1 entries. O(m+n) time, O(1) space.

Time
O(m+n)
Space
O(1)
function merge(nums1, m, nums2, n) {
  let i = m - 1, j = n - 1, k = m + n - 1;
  while (j >= 0) {
    if (i >= 0 && nums1[i] > nums2[j]) nums1[k--] = nums1[i--];
    else nums1[k--] = nums2[j--];
  }
}

Tradeoff:

Chegg-specific tips

Chegg interviewers expect the back-to-front merge because their nightly rating-rollup job merges sorted spans without growing the source buffer in memory.

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