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9. Merge Sorted Array

easyAsked at Expedia

Merge nums2 into nums1 in-place so the result is sorted.

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 nums1 and nums2 into a single array sorted in non-decreasing order. nums1 has length m+n with the last n elements set to 0.

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 onto end, sort full 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. Three-pointer reverse merge

Fill nums1 from the back so we don't overwrite. Expedia uses this when merging back-end sorted hotel availability into a primary inventory cache.

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:

Expedia-specific tips

Expedia interviewers grade O(m+n) explicitly; walking forward and shifting is a red flag that signals you missed the back-fill trick.

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