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8. Merge Sorted Array

easyAsked at Roblox

Merge two sorted arrays in-place into the first array.

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

Problem

You are given two sorted integer arrays nums1 (length m+n) and nums2 (length n). nums1 has m valid elements followed by n empty slots. 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. Concatenate and sort

Copy nums2 in 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. Three pointers from the back

Compare from the end and write into nums1's tail, avoiding overwriting unread values.

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:

Roblox-specific tips

Roblox engineers like the tail-pointer trick because it generalizes to merging chunked physics-state updates without extra allocations.

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