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8. Merge Sorted Array

easyAsked at DigitalOcean

Merge two sorted arrays in-place into the first one — DigitalOcean uses this to test two-pointer fluency that maps to merging per-region usage windows into one billing batch.

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

Problem

Given two sorted arrays nums1 of length m+n with m valid elements and nums2 of length n, merge nums2 into nums1 as one sorted array in-place. Use the trailing n empty slots in nums1.

Constraints

  • nums1.length == m+n
  • nums2.length == n
  • 0 <= m, n <= 200
  • Both inputs 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 and sort

Overwrite tail with nums2, then 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. Backwards two-pointer

Fill nums1 from the tail using the larger of the two pointers. No shifting needed.

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:

DigitalOcean-specific tips

DigitalOcean grades the tail-walk solution highly because it avoids the destructive sort that would re-touch all m+n entries during high-volume usage merges.

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