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4. Remove Duplicates from Sorted Array

easyAsked at GitLab

In-place deduplicate a sorted array 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 duplicates in-place so each unique element appears once. Return the new length k; the first k elements of nums must be the unique values in order.

Constraints

  • 1 <= nums.length <= 3*10^4
  • Must be O(1) extra space

Examples

Example 1

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

Example 2

Input
[0,0,1,1,1,2,2,3,3,4]
Output
5, nums=[0,1,2,3,4,...]

Approaches

1. Set + rewrite

Use a Set then copy values 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 pointers

Slow pointer marks next unique slot; fast scans for new values.

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:

GitLab-specific tips

GitLab cares about in-place mutations because pipeline runners care about memory ceilings — call out the O(1) extra space budget explicitly.

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