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4. Remove Duplicates from Sorted Array

easyAsked at GitHub

Remove duplicates in-place from a sorted array — GitHub uses this as the entry-level analogue to dedup-ing sorted SHA streams in the packfile delta index.

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

Problem

Given an integer array nums sorted in non-decreasing order, remove duplicates in-place such that each unique element appears only once. Return the number of unique elements.

Constraints

  • 1 <= nums.length <= 3 * 10^4
  • -100 <= nums[i] <= 100
  • nums is sorted

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

Use a Set, rebuild array.

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-pointer in-place

Maintain a write pointer; advance it only when the current value differs from the last written value. O(1) extra space exploits the sorted invariant.

Time
O(n)
Space
O(1)
function removeDuplicates(nums) {
  if (nums.length === 0) return 0;
  let write = 1;
  for (let read = 1; read < nums.length; read++) {
    if (nums[read] !== nums[read - 1]) {
      nums[write++] = nums[read];
    }
  }
  return write;
}

Tradeoff:

GitHub-specific tips

GitHub grades whether you exploit the sorted invariant — the same trick used in pack-index binary search over sorted SHAs.

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