Skip to main content

4. Remove Duplicates from Sorted Array

easyAsked at Klarna

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

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, 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. Use a Set

Insert into Set then copy 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 next write index; fast pointer scans. Sorted property guarantees correctness.

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:

Klarna-specific tips

Klarna interviewers reward in-place rewriting since merchant-side daily settlement files dedupe sorted transaction IDs without allocating shadow buffers.

Solve it now

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

Output

Press Run or Cmd+Enter to execute

Practice these live with InterviewChamp.AI

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

Practice these live with InterviewChamp.AI →