Skip to main content

4. Remove Duplicates from Sorted Array

easyAsked at Coupang

In-place dedupe of a sorted array — Coupang asks this to test pointer-pair fluency before SKU-list deduplication problems.

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

Problem

Given an integer array nums sorted in non-decreasing order, remove the duplicates in-place such that each unique element appears only once. Return the number of unique elements k.

Constraints

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

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. Brute force

Build a Set, copy back.

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 marks write position; fast pointer scans.

Time
O(n)
Space
O(1)
function removeDuplicates(nums) {
  if (nums.length === 0) 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:

Coupang-specific tips

Coupang weighs in-place O(1) space heavily because their warehouse SKU pipelines run on memory-constrained edge nodes.

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 Coupang interview questions under real-loop conditions with instant feedback on your reasoning, complexity claims, and code.

Practice these live with InterviewChamp.AI →