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4. Remove Duplicates from Sorted Array

easyAsked at Roblox

Remove duplicates from a sorted array in-place and return the new length.

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

Problem

Given a sorted integer array nums, remove duplicates in-place such that each unique element appears only once. Return the count k of unique elements; the first k slots of nums must contain those elements in order.

Constraints

  • 1 <= nums.length <= 3 * 10^4
  • nums is 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 then refill

Use a Set to dedupe then write back.

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

Slow pointer marks write index, fast pointer scans. Advance slow only on a new value.

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

Tradeoff:

Roblox-specific tips

Roblox values in-place edits — keeping a single buffer matters when you are compacting a player's inventory snapshot on a memory-constrained server.

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