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4. Remove Duplicates from Sorted Array

easyAsked at MercadoLibre

Remove duplicates from a sorted array in place and return the new length.

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

Problem

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

Constraints

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

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. Brute force

Use a Set then write back.

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

Write pointer advances only when current differs from previous unique.

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

Tradeoff:

MercadoLibre-specific tips

MercadoLibre likes seeing in-place writes here because their listing-feed dedup runs on huge sorted blocks where allocation cost matters.

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