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4. Remove Duplicates from Sorted Array

easyAsked at Lyft

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 duplicates in-place so each unique element appears only once. Return the count of unique elements; the first k slots of nums must hold them in order.

Constraints

  • 1 <= nums.length <= 3*10^4
  • Array is sorted ascending

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

Dedupe with a Set then copy back.

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 next write slot; fast pointer scans. Only writes when value differs.

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[i - 1]) {
      nums[k++] = nums[i];
    }
  }
  return k;
}

Tradeoff:

Lyft-specific tips

Lyft pushes you to mutate in place when answering — they care about memory pressure on mobile clients that ship dedup logic for cached ride history.

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Output

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