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9. Merge Sorted Array

easyAsked at Udemy

Merge two sorted arrays into the first in place — Udemy uses this to test reverse-pointer thinking before merging course-result feeds.

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

Problem

You are given two sorted integer arrays nums1 (size m+n with m valid elements then zeros) and nums2 (size n). Merge nums2 into nums1 as one sorted array, in place.

Constraints

  • nums1.length == m + n
  • 0 <= m, n <= 200
  • nums1 and nums2 are sorted non-decreasing

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 into nums1's tail and 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. Reverse three-pointer

Fill from the back. Compare last valid in nums1 and last in nums2; write the larger to the end.

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:

Udemy-specific tips

Udemy interviewers may reframe this as merging two ranked search-result pages from different shards — show you avoid extra allocation.

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Output

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