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4. Remove Duplicates from Sorted Array

easyAsked at CircleCI

Remove duplicates in-place from 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 the duplicates in-place such that each unique element appears only once. The relative order of the elements should be kept the same and return the number of unique elements.

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. Set + rewrite

Build a Set, write distinct values back into nums.

Time
O(n)
Space
O(n)
const s = new Set(nums);
let i = 0;
for (const v of s) nums[i++] = v;
return s.size;

Tradeoff:

2. Two pointers

Keep a write index; advance only when the next element differs.

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:

CircleCI-specific tips

CircleCI prefers the constant-space variant — they reuse this pattern for compacting build-cache manifests after dedup.

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