Skip to main content

4. Remove Duplicates from Sorted Array

easyAsked at Quora

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

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

Problem

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

Constraints

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

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

Materialize 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 pointers in place

Slow pointer marks last unique slot; fast pointer scans.

Time
O(n)
Space
O(1)
function removeDuplicates(nums) {
  if (!nums.length) 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:

Quora-specific tips

Quora favors in-place two-pointer scans because their question-dedup jobs run over sorted-by-hash batches where extra memory per row would blow up the offline index.

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

Practice these live with InterviewChamp.AI →