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4. Remove Duplicates from Sorted Array

easyAsked at Glassdoor

Dedupe a sorted array in place — Glassdoor uses this to test two-pointer reasoning on the kind of stream they dedupe in salary reports.

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 and return the new length. Do not allocate extra space for another array.

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. Set + rewrite

Push to a Set then rewrite nums in place.

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

Tradeoff:

2. Two pointers

Slow pointer writes uniques; fast pointer scans.

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

Tradeoff:

Glassdoor-specific tips

Glassdoor grades for the explicit O(1)-space contract — they dedupe billions of review tuples in stream pipelines and want candidates who naturally reach for in-place writes.

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