Skip to main content

61. Remove Duplicates from Sorted Array II

mediumAsked at Plaid

Remove duplicates in-place such that each element appears at most twice. Plaid asks this as a two-pointer variant — exactly the shape they use to enforce 'at most 2 retries per transaction in the same minute' logs.

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

Source citations

Public interview reports confirming this problem appears in Plaid loops.

  • LeetCode Discuss (2026)— Plaid OA two-pointer.
  • Glassdoor (2025)— Plaid SWE II screen.

Problem

Given an integer array nums sorted in non-decreasing order, remove some duplicates in-place such that each unique element appears at most twice. The relative order of the elements should be kept the same. Return k after placing the final result in the first k slots of nums.

Constraints

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

Examples

Example 1

Input
nums = [1,1,1,2,2,3]
Output
5, nums = [1,1,2,2,3,_]

Example 2

Input
nums = [0,0,1,1,1,1,2,3,3]
Output
7, nums = [0,0,1,1,2,3,3,_,_]

Approaches

1. Count occurrences with Map

Count occurrences, then rebuild with at most 2 of each.

Time
O(n)
Space
O(n)
// Works but uses extra space and ignores sortedness.

Tradeoff: Doesn't exploit the sorted input.

2. Two-pointer with k-tolerance

Generalizable to 'at most k duplicates'. Write pointer trails. If nums[read] != nums[write - k], write.

Time
O(n)
Space
O(1)
function removeDuplicates(nums) {
  const k = 2;
  let write = 0;
  for (const n of nums) {
    if (write < k || nums[write - k] !== n) {
      nums[write++] = n;
    }
  }
  return write;
}

Tradeoff: Generalizes to any k. The check 'nums[write - k] !== n' verifies the candidate is different from the k-th-most-recently-written, which guarantees at most k of any value.

Plaid-specific tips

Plaid loves the k-generalized version because it shows you've abstracted beyond k=2. Bonus signal: explain why nums[write - k] is the right comparison — it's the value that would be the (k+1)th copy if we wrote n. Connect to rate-limiting logs where each user can have at most k retries per minute.

Common mistakes

  • Hard-coding k=2 with a counter — fine but doesn't show generalization.
  • Comparing nums[read] to nums[read-1] — doesn't capture 'at most 2 in window'.
  • Off-by-one on write index when k > write.

Follow-up questions

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

  • At most k duplicates (parameter).
  • Same problem on a linked list (LC 82).
  • Stream version with bounded memory.

Solve it now

Free. No sign-up. Python and JavaScript run instantly in your browser.

Output

Press Run or Cmd+Enter to execute

FAQ

Why nums[write - k] and not nums[write - 1]?

Comparing to write-1 only catches the immediate predecessor; we'd allow [1,1,1,1] because each new 1 differs from no predecessor. write-k catches the k-th previous.

What if the array has < k elements?

The 'write < k' check writes everything without comparison.

Practice these live with InterviewChamp.AI

Drill Remove Duplicates from Sorted Array II and other Plaid interview questions under real-loop conditions with instant feedback on your reasoning, complexity claims, and code.

Practice these live with InterviewChamp.AI →