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9. Merge Sorted Array

easyAsked at Coursera

Merge two sorted arrays in-place — Coursera tests back-fill two-pointer technique for in-place enrollment merges.

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

Problem

Given nums1 of length m+n with m valid elements then n zeros, and nums2 of length n, merge nums2 into nums1 in-place so nums1 becomes a sorted union.

Constraints

  • 0 <= m, n <= 200
  • 1 <= m + n <= 200
  • nums1.length == m + n

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

Approaches

1. Concat + sort

Drop nums2 into the zeros, sort the whole array.

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. Back-fill two pointers

Walk from the tail; place the larger value at index m+n-1.

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:

Coursera-specific tips

Coursera reviewers will probe whether you can keep the merge O(1) extra memory — they batch-merge enrollment shards in services that can't afford an allocation per merge.

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