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4. Remove Duplicates from Sorted Array

easyAsked at Nubank

Deduplicate a sorted list of transaction IDs in-place to prepare a reconciliation batch.

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 number of unique elements; the first k slots must hold the unique values.

Constraints

  • 1 <= nums.length <= 3 * 10^4
  • 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 rewrite

Collect uniques in a Set, write back.

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

Tradeoff:

2. Two pointers

Write pointer trails read pointer; advance write only on new value.

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

Tradeoff:

Nubank-specific tips

Nubank cares that you can spot when an in-place pattern keeps the JVM GC pressure off a high-volume ledger job.

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