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8. Merge Sorted Array

easyAsked at GitLab

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

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

Problem

Given nums1 of length m+n (with m valid elements then n zeros) and nums2 of length n, merge them so nums1 becomes sorted in non-decreasing order with all m+n elements. Mutate in place.

Constraints

  • 0 <= m,n <= 200
  • Must mutate nums1
  • Both inputs are sorted

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 then sort

Copy nums2 over zeros 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. Three pointers from the back

Fill from end of nums1 to avoid overwriting unread values.

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:

GitLab-specific tips

GitLab will probe whether your fill-from-back invariant holds when one array is exhausted — narrate the j>=0 stop condition explicitly.

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Output

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