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4. Remove Duplicates from Sorted Array

easyAsked at Booking

Deduplicate a sorted array in-place — Booking checks this to ensure you can collapse duplicate availability rows efficiently.

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

Problem

Given a sorted array, remove duplicates in-place so each element appears once and return the new length. Modify the array such that the first k elements contain unique values.

Constraints

  • 1 <= nums.length <= 3 * 10^4
  • Array 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. Brute force set

Use a Set then overwrite — but violates the in-place rule.

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 last unique; fast scans ahead and overwrites when a new value appears.

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:

Booking-specific tips

Booking cares about memory efficiency when streaming room-availability snapshots — call out that the two-pointer pattern lets you dedupe a supplier feed in place.

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Output

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