Skip to main content

4. Remove Duplicates from Sorted Array

easyAsked at Brex

Remove duplicates in-place from a sorted array 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. Return the number of unique elements; the first k slots of nums must hold the unique values.

Constraints

  • 1 <= nums.length <= 3 * 10^4
  • -100 <= nums[i] <= 100
  • nums 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. Brute force with Set

Push into a Set, then write unique values back.

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

Tradeoff:

2. Two pointers in-place

Write pointer lags; advance only when next value differs. Truly in-place with O(1) extra memory.

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];
      k++;
    }
  }
  return k;
}

Tradeoff:

Brex-specific tips

Brex graders care about in-place mutation here because they map it to deduping batched card transactions before posting to the ledger, so be explicit about not allocating a parallel buffer.

Solve it now

Free. No sign-up. Python and JavaScript run instantly in your browser.

Output

Press Run or Cmd+Enter to execute

Practice these live with InterviewChamp.AI

Drill Remove Duplicates from Sorted Array and other Brex interview questions under real-loop conditions with instant feedback on your reasoning, complexity claims, and code.

Practice these live with InterviewChamp.AI →