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8. Merge Sorted Array

easyAsked at Slack

Merge nums2 into nums1 in-place as one sorted array.

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. Merge nums2 into nums1 as one sorted array in-place; nums1 has length m+n with the last n slots set to 0.

Constraints

  • nums1.length == m + n
  • 0 <= m, n <= 200
  • 1 <= 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=[1], m=1, nums2=[], n=0
Output
[1]

Approaches

1. Concat and sort

Copy nums2 into the tail, 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 pointers

Walk from end of both arrays into the tail of nums1, placing the larger value each step. No extra 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:

Slack-specific tips

Slack message-buffer reconciliation after socket reconnect is exactly this — they look for the backwards-walk insight, not just any merge.

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