Skip to main content

9. Merge Sorted Array

easyAsked at Duolingo

Merge two sorted integer arrays into nums1 in-place.

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

Problem

You are given two sorted integer arrays nums1 and nums2 of sizes m and n. Merge nums2 into nums1 as one sorted array; nums1 has length m + n with the last n slots set to zero placeholders.

Constraints

  • nums1.length == m + n
  • nums2.length == n
  • 0 <= m, n <= 200
  • Both inputs are sorted ascending

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 the back of nums1 then sort.

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 pointers from the back

Place the larger of nums1[i] vs nums2[j] at nums1[k] starting from the end; no temporary buffer needed.

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:

Duolingo-specific tips

Duolingo merges newly-due cards with the existing review queue dozens of times per session; the three-pointer back-walk avoids allocating on slow mobile devices.

Solve it now

Free. No sign-up. Python and JavaScript run instantly in your browser.

Output

Press Run or Cmd+Enter to execute

Practice these live with InterviewChamp.AI

Drill Merge Sorted Array and other Duolingo interview questions under real-loop conditions with instant feedback on your reasoning, complexity claims, and code.

Practice these live with InterviewChamp.AI →