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4. Remove Duplicates from Sorted Array

easyAsked at Freshworks

Deduplicate ticket IDs streaming in sorted order without allocating a second buffer.

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

Problem

Given a sorted integer array, remove duplicates in place such that each unique element appears once. Return the new length k. The first k elements of the array must hold the unique values in original order.

Constraints

  • 1 <= nums.length <= 3 * 10^4
  • Array is sorted non-decreasing
  • Must mutate in place

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

Copy uniques into a Set then back into the array.

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

Tradeoff:

2. Two pointers

Slow pointer marks the next unique slot; fast pointer scans. Write only when nums[fast] differs from nums[slow-1].

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

Tradeoff:

Freshworks-specific tips

Freshworks frames this as deduping ticket IDs in a sorted queue — call out the in-place constraint up front since their SaaS backend hates extra allocations on hot paths.

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