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4. Remove Duplicates from Sorted Array

easyAsked at Udemy

Dedupe a sorted array in place — Udemy uses this to gauge two-pointer hygiene before dedupe questions on course catalog imports.

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

Problem

Given a sorted integer array nums, remove duplicates in place so each unique element appears once. Return the count k of unique elements; the first k elements of nums must hold those uniques.

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. Extra set

Copy uniques into a Set, then write them back.

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

Tradeoff:

2. Two-pointer in place

Keep a write pointer k. Advance read pointer i; when nums[i] differs from nums[k-1], write it forward.

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

Tradeoff:

Udemy-specific tips

Udemy interviewers will pivot to deduping repeat course uploads after a CSV import — show you can adapt the two-pointer pattern to record-level dedupe keys.

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