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8. Merge Sorted Array

easyAsked at Ola

Merge two sorted 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. Merge nums2 into nums1 as one sorted array; nums1 has length m + n with the last n positions reserved as zeros.

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. Append and sort

Copy nums2 into the tail and sort the whole array.

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

Write largest remaining element at the back of nums1 each step; no extra space needed.

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:

Ola-specific tips

Ola favors the reverse three-pointer because it mirrors how ETA windows are merged from upstream supply streams; mention why writing from the end avoids shifting.

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