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4. Remove Duplicates from Sorted Array

easyAsked at Expedia

Remove duplicates from a sorted array in-place 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. 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, then write back.

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

Tradeoff:

2. Two pointers in-place

Slow pointer marks unique slot; fast pointer scans. Expedia uses this to dedupe sorted hotel rate-plans before display.

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:

Expedia-specific tips

Expedia favors in-place O(1) space solutions when discussing memory-bound supplier-feed normalization — call that out explicitly.

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