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8. Merge Sorted Array

easyAsked at Zoom

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

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

Problem

Two sorted arrays nums1 and nums2 are given with lengths m and n. nums1 has size m+n where the last n cells are zeros. Merge nums2 into nums1 as one sorted array, in place.

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

Approaches

1. Concat and sort

Copy nums2 in, 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. Three-pointer from the back

Fill from the rear so we never overwrite unprocessed nums1 cells.

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:

Zoom-specific tips

Zoom's audio-mix layer merges packet buffers exactly this way (back-to-front to avoid overwriting late-arriving frames), so call out the production parallel.

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