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4. Remove Duplicates from Sorted Array

easyAsked at N26

Remove duplicates in-place from a sorted array and return the new length. N26 uses this for collapsing repeated transactions returned by upstream ledgers.

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

Problem

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

Constraints

  • 1 <= nums.length <= 3*10^4
  • nums is sorted non-decreasing
  • In-place modification only

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

Build a Set and rewrite the prefix.

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 next write slot; fast scans forward.

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:

N26-specific tips

N26 expects you to mention idempotency: collapsing duplicate webhook deliveries from card-network partners uses this same dedup pattern.

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