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8. Merge Sorted Array

easyAsked at Spotify

Merge nums2 into nums1 in place where nums1 has space at the end.

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

Problem

Given two sorted integer arrays nums1 (size m+n with last n slots empty) and nums2 (size n), 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 + sort

Copy nums2 into nums1 tail then sort the whole thing

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. Reverse two pointers

Fill from the back: largest remaining of m or n goes into the tail slot. Avoids overwriting unread data.

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:

Spotify-specific tips

Spotify's playlist-merge service relies on this exact reverse-pointer technique when combining personalized and editorial picks without allocating an extra buffer per request.

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