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8. Merge Sorted Array

easyAsked at Tesla

Merge two sorted arrays in-place, where the first has extra trailing space.

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

Problem

nums1 has length m + n with the first m sorted elements followed by zeros. nums2 has n sorted elements. Merge nums2 into nums1 in-place so nums1 is sorted.

Constraints

  • nums1.length === m + n
  • 0 <= m, n <= 200
  • nums1, nums2 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. Copy and sort

Splice nums2 in, 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-pointer from back

Fill from rightmost slot using two read pointers — no shifting.

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:

Tesla-specific tips

Tesla autopilot data fusion merges timestamped sensor streams the same way — Tesla wants the back-to-front pointer trick that avoids shifting and double allocations.

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