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4. Remove Duplicates from Sorted Array

easyAsked at Ramp

In-place dedupe a sorted array; return the new length.

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

Problem

Given a sorted array nums, remove duplicates in-place such that each unique value appears once. Return the count of unique elements; the first k positions of nums must hold them in order.

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 then copy

Build set of uniques, copy back to 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 next write slot; fast pointer scans. Write only when new value encountered. O(1) extra space.

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:

Ramp-specific tips

Ramp interviewers reframe this as deduping recurring vendor invoice IDs in a sorted nightly batch, so call out idempotency and stable order of the first occurrence.

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