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8. Merge Sorted Array

easyAsked at Monzo

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

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

Problem

You are given two integer arrays nums1 and nums2, sorted in non-decreasing order, and two integers m and n, representing the number of elements in nums1 and nums2 respectively. Merge nums2 into nums1 as one sorted array in-place; nums1 has size 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. Concat and sort

Copy nums2 in, then sort.

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. Two pointers from end

Fill nums1 from the back to avoid overwriting unread cells.

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) {
    nums1[k--] = (i >= 0 && nums1[i] > nums2[j]) ? nums1[i--] : nums2[j--];
  }
}

Tradeoff:

Monzo-specific tips

Monzo loves the back-to-front trick — it shows you can reason about in-place state, which is critical for their immutable-but-streaming ledger code.

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Output

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