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8. Merge Sorted Array

easyAsked at Lyft

Merge two sorted integer arrays in-place into the first one.

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

Problem

You are given two integer arrays nums1 and nums2, sorted in non-decreasing order, with sizes m and n. Merge nums2 into nums1 in-place so nums1 becomes one sorted array of length m+n. nums1 has trailing zeros to accommodate this.

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 = [0], m = 0, nums2 = [1], n = 1
Output
[1]

Approaches

1. Append + sort

Copy nums2 into the tail and sort nums1.

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

Walk both arrays from the end, write the larger value into the tail of nums1. No extra space.

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:

Lyft-specific tips

Lyft uses this to test you on in-place algorithms — they merge per-region trip queues in memory and bonus signal is recognizing that writing from the back avoids overwrite bugs.

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