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9. Merge Sorted Array

easyAsked at CircleCI

Merge two sorted arrays in-place into the first one.

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

Problem

You are given two integer arrays nums1 and nums2, sorted in non-decreasing order. Merge nums2 into nums1 as one sorted array in-place. nums1 has length m + n with the last n positions reserved for nums2.

Constraints

  • nums1.length == m + n
  • nums2.length == n
  • 0 <= m, n <= 200

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

Copy nums2 into nums1's tail, then 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 two-pointer

Fill nums1 from the back so we never overwrite unread data.

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:

CircleCI-specific tips

CircleCI checks that you don't allocate auxiliary buffers — they prize the reverse merge for in-place log-event buffer compaction.

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