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4. Remove Duplicates from Sorted Array

easyAsked at GoDaddy

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

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

Problem

Given a sorted array nums, remove the duplicates in-place such that each unique element appears only once. Return the number of unique elements, with the first k positions of nums holding them in order.

Constraints

  • 1 <= nums.length <= 3*10^4
  • -100 <= nums[i] <= 100
  • nums 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 then copy

Use a Set to dedupe, then write back into nums.

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

Slow pointer marks last unique; fast scans and writes new uniques.

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:

GoDaddy-specific tips

GoDaddy uses this to test whether candidates can dedupe domain-name lists (e.g., suggested alternates) in-place without re-allocating large arrays.

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Output

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