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4. Remove Duplicates from Sorted Array

easyAsked at SoFi

Dedupe a sorted array in-place — SoFi uses this as a stand-in for deduping repeated loan-disbursement records.

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

Problem

Given a sorted integer array, remove duplicates in-place so each element appears once, and return the new length.

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. Brute force

Use a Set, rebuild array.

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

Tradeoff:

2. Two pointers

Write pointer lags read pointer, copying only when value changes. O(1) extra space — SoFi's expected answer.

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:

SoFi-specific tips

SoFi rewards candidates who write in-place with O(1) extra space and connect it to streaming dedupe of duplicate-payment events on their ledger.

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Output

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