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4. Remove Duplicates from Sorted Array

easyAsked at Adyen

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

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

Problem

Given a sorted array nums, remove duplicates in-place so each element appears once. Return the count of unique elements; ordering of the first k elements must be preserved.

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

Collect unique values then overwrite.

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 pointers in-place

Slow pointer marks the next unique slot; fast pointer scans.

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

Tradeoff:

Adyen-specific tips

Adyen rewards O(1) auxiliary space because their high-throughput transaction enrichment pipeline cannot afford per-row allocations at peak Black Friday volumes.

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