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8. Merge Sorted Array

easyAsked at Canva

Merge two sorted arrays in place — Canva uses this to test whether you can avoid scratch buffers when combining template asset lists.

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

Problem

Given two sorted arrays nums1 (length m + n with trailing zeros) and nums2 (length n), merge nums2 into nums1 in non-decreasing order, 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 and sort

Overwrite the tail, sort the array.

Time
O((n+m) log (n+m))
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 from the back

Fill from the end of nums1 using two read pointers and one write pointer to avoid overwriting valid data.

Time
O(n + m)
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:

Canva-specific tips

Canva interviewers reward backward-iteration tricks because their editor's undo/redo stacks merge similarly without scratch allocations.

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