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4. Remove Duplicates from Sorted Array

easyAsked at Tesla

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

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

Problem

Given a sorted integer array nums, remove duplicates in-place so each element appears once and return the number of unique elements. The first k positions must hold the unique values.

Constraints

  • 1 <= nums.length <= 3 * 10^4
  • Array is 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. Set + rewrite

Dump into a Set, write 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-pointer in-place

Slow pointer marks the last unique slot; fast scans forward.

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:

Tesla-specific tips

Tesla cares deeply about in-place memory ops — extra allocation in a control loop can cost a frame of latency and miss a real-time deadline.

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Output

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