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8. Merge Sorted Array

easyAsked at Yelp

Merge nums2 into nums1 in-place — Yelp's analog for combining two sorted batches of incoming reviews into a single chronological feed without extra memory.

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

Problem

You are given two sorted integer arrays nums1 and nums2 of lengths m and n. nums1 has size m + n, with the last n slots set to 0 as padding. Merge nums2 into nums1 in-place in sorted order.

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. Append + sort

Overwrite the tail and 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. Three pointers from the back

Fill from the tail using the larger of each cursor.

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:

Yelp-specific tips

Yelp rewards the backward three-pointer trick and follows up by asking how you'd merge new-review batches into the sorted business detail feed during a hot-path write.

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