Skip to main content

4. Remove Duplicates from Sorted Array

easyAsked at Mercury

Remove duplicates in-place from a sorted array and return new length.

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

Problem

Given a sorted array nums, remove duplicates in-place such that each unique element appears only once. Return the new length, and the first k elements of nums should hold the unique values.

Constraints

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

Examples

Example 1

Input
nums = [1,1,2]
Output
k=2, nums=[1,2,_]

Example 2

Input
nums = [0,0,1,1,1,2,2,3,3,4]
Output
k=5, nums=[0,1,2,3,4,_,_,_,_,_]

Approaches

1. Set + rebuild

Dump to a Set, copy back into nums.

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

Tradeoff:

2. Two-pointer overwrite

Slow pointer holds next-write index; fast scans and writes only when value changes.

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:

Mercury-specific tips

At Mercury, this models dedup of ACH return notifications that the partner bank may re-emit on retry, so emphasize idempotent ledger writes keyed on the trace number.

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 Mercury interview questions under real-loop conditions with instant feedback on your reasoning, complexity claims, and code.

Practice these live with InterviewChamp.AI →