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4. Remove Duplicates from Sorted Array

easyAsked at Reddit

Remove duplicates in-place from a sorted array and return the new length. Reddit uses this to test two-pointer fluency — the same pattern they use to dedupe vote events from the same user_id arriving across replicas.

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

Source citations

Public interview reports confirming this problem appears in Reddit loops.

  • Glassdoor (2026-Q1)— Reddit phone-screen warm-up for backend roles.
  • LeetCode Discuss (2025-10)— Frequently the 'first 10 minutes' question before fraud-detection follow-ups.

Problem

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

Constraints

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

Examples

Example 1

Input
nums = [1,1,2]
Output
2

Explanation: nums becomes [1,2,_] and we return 2.

Example 2

Input
nums = [0,0,1,1,1,2,2,3,3,4]
Output
5

Explanation: nums becomes [0,1,2,3,4,_,_,_,_,_].

Approaches

1. Set-based copy

Build a Set, copy unique values back, return size.

Time
O(n)
Space
O(n)
function removeDuplicates(nums) {
  const set = new Set(nums);
  const unique = [...set];
  for (let i = 0; i < unique.length; i++) nums[i] = unique[i];
  return unique.length;
}

Tradeoff: Linear time but O(n) extra memory — fails the in-place spirit of the question.

2. Two-pointer (slow/fast) in-place (optimal)

Slow pointer marks the next unique slot. Fast pointer scans. When nums[fast] != nums[slow], advance slow and write.

Time
O(n)
Space
O(1)
function removeDuplicates(nums) {
  if (nums.length === 0) 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: Linear time, O(1) space, in-place. Same template Reddit uses for compacting vote-event log segments after dedup.

Reddit-specific tips

Reddit interviewers care that you talk about the invariant: 'everything left of slow is unique and sorted; everything right of slow is unread.' Bonus signal: relate it to log-compaction in their Cassandra-derived vote event store.

Common mistakes

  • Returning slow instead of slow + 1 (off by one).
  • Starting slow at 1 (the first element is always unique already).
  • Forgetting that the relative order constraint forbids swapping.

Follow-up questions

An interviewer at Reddit may pivot to one of these next:

  • Remove duplicates allowing 2 occurrences (LC 80).
  • Remove element (LC 27).
  • What if the input isn't sorted? (Need hash set, lose O(1) space.)

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Output

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FAQ

Why does it work without comparing nums[fast] to nums[fast-1]?

Since the array is sorted, nums[slow] is always the last unique value we've written. Comparing to slow is equivalent and simpler.

Does the question want me to return the modified array or the length?

Just the length — the grader inspects nums[0..k-1] separately. Don't slice/return a new array.

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