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4. Remove Duplicates from Sorted Array

easyAsked at Activision

Remove duplicates in-place from a sorted array — Activision uses this to test two-pointer fluency on leaderboard scrubbing.

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

Problem

Given a sorted integer array, remove duplicates in-place such that each unique element appears once and return the new length. Order of remaining elements must be preserved.

Constraints

  • 1 <= nums.length <= 3 * 10^4
  • -100 <= nums[i] <= 100
  • 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. Set then rewrite

Push uniques into a Set, then write back.

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

Tradeoff:

2. Two-pointer in-place

Slow pointer marks the next write slot; fast pointer scans. Only write when value changes.

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:

Activision-specific tips

Activision wants O(1) space — leaderboards run at scale and they'll dock you for allocating an auxiliary Set when two pointers do the job.

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Output

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