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8. Merge Sorted Array

easyAsked at Box

Merge nums2 into nums1 in place — Box uses this when reconciling two sorted change journals during a sync rebase.

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

Problem

Given two integer arrays nums1 and nums2 sorted in non-decreasing order, and two integers m and n representing the number of elements in each, merge nums2 into nums1 as one sorted array. The final sorted array should be stored inside nums1, which has length m+n.

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. Copy and sort

Copy nums2 into the tail of nums1, 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 end

Fill nums1 from the back using two read pointers; 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:

Box-specific tips

Box wants the in-place backward merge — they tell candidates this is the same pattern they use when journaling sync deltas in pre-allocated buffers.

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