4. Remove Duplicates from Sorted Array

easyAsked at Canva

Compact a sorted array in place by removing duplicates — Canva uses this to evaluate two-pointer reasoning relevant to deduping asset-IDs in a design.

By Alex Chen, Founder, InterviewChamp.AI · Last verified 2026-05-20

Problem

Given a sorted integer array nums, remove the duplicates in place such that each unique element appears only once. Return the number of unique elements k; the first k elements of nums should hold the result.

Constraints

1 <= nums.length <= 3 * 10^4
nums is sorted non-decreasing

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 with Set

Copy to a Set, write back unique values.

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-pointer in place

Slow pointer marks next unique slot; fast pointer scans forward. Write only when distinct. O(1) extra space.

Time: O(n)
Space: O(1)

function removeDup(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:

Canva-specific tips

Canva interviewers care that you keep memory in-place when designs scale to thousands of objects, mirroring how their editor compresses element arrays.

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Output

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