Skip to main content

4. Remove Duplicates from Sorted Array

easyAsked at Etsy

Compact a sorted array in-place — Etsy uses it to check whether you write O(1)-space code without leaning on a Set.

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

Problem

Given an integer array nums sorted in non-decreasing order, remove duplicates in-place so that each unique element appears once. Return the new length k; the first k elements of nums must hold the unique values in original order.

Constraints

  • 1 <= nums.length <= 3 * 10^4
  • -100 <= nums[i] <= 100
  • nums is sorted ascending

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. Use a Set

Copy unique values into a Set, then back into the array.

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

Tradeoff:

2. Two pointers

Keep a write pointer that advances only on new values.

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

Tradeoff:

Etsy-specific tips

Etsy interviewers grade on whether you mutate in-place — they're alert to candidates who'd waste memory deduping recommended-listing arrays at scale.

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 Remove Duplicates from Sorted Array and other Etsy interview questions under real-loop conditions with instant feedback on your reasoning, complexity claims, and code.

Practice these live with InterviewChamp.AI →