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4. Remove Duplicates from Sorted Array

easyAsked at Monzo

Remove duplicate transaction IDs in-place from a sorted array.

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

Problem

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

Constraints

  • 1 <= nums.length <= 3 * 10^4
  • 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

Approaches

1. Set + rewrite

Build a set, then copy back. Wastes O(n) space.

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

Tradeoff:

2. Two pointers

Slow pointer for next write slot; fast pointer scans.

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:

Monzo-specific tips

Monzo asks in-place problems to see if you respect memory budgets — important for their high-throughput ledger pipelines.

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Output

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