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4. Remove Duplicates from Sorted Array

easyAsked at Squarespace

Dedup in-place on a sorted array and return the new length; Squarespace uses it to test two-pointer fluency.

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

Problem

Given a sorted integer array nums, remove duplicates in-place such that each unique value appears once. Return the new length k.

Constraints

  • 1 <= nums.length <= 3*10^4
  • Sorted in non-decreasing order

Examples

Example 1

Input
nums=[1,1,2]
Output
2, nums=[1,2,_]

Example 2

Input
nums=[0,0,1,1,1,2,2,3,3,4]
Output
5, nums=[0,1,2,3,4,...]

Approaches

1. Set + rewrite

Push to Set, copy back, slice.

Time
O(n)
Space
O(n)
const s=[...new Set(nums)]; for(let i=0;i<s.length;i++) nums[i]=s[i]; return s.length;

Tradeoff:

2. Two pointers

Slow pointer writes uniques, fast pointer scans.

Time
O(n)
Space
O(1)
function removeDuplicates(nums) {
  if (!nums.length) return 0;
  let k = 1;
  for (let i = 1; i < nums.length; i++) {
    if (nums[i] !== nums[i - 1]) nums[k++] = nums[i];
  }
  return k;
}

Tradeoff:

Squarespace-specific tips

Squarespace looks for in-place edits with zero extra allocation — they reason about it the same way they trim duplicate asset references in a published site.

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Output

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