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8. Merge Sorted Array

easyAsked at Glassdoor

Merge nums2 into nums1 in place — Glassdoor uses this to test reverse-pointer thinking on a problem with hidden cleverness.

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 zero.

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 + sort

Overwrite the tail with nums2 and 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 pointers, end-first

Fill nums1 from the right so no element is overwritten before use.

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:

Glassdoor-specific tips

Glassdoor grades for the in-place reverse pointer — they care about engineers who instinctively merge sorted streams without extra allocation, mirroring their salary-record aggregation jobs.

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