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4. Remove Duplicates from Sorted Array

easyAsked at Instacart

Compact a sorted array in place so each value appears once — Instacart frames this as dedupping a sorted SKU scan from a shopper's bag.

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

Problem

Given a sorted integer array nums, remove duplicates in place such that each unique element appears only once. Return the new length k; the first k elements of nums must hold the unique values.

Constraints

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

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 then copy

Drop into a Set, sort the unique values back into nums.

Time
O(n log 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 pointers in place

Slow pointer marks the next unique slot, fast pointer scans for distinct values.

Time
O(n)
Space
O(1)
function removeDuplicates(nums) {
  if (!nums.length) 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:

Instacart-specific tips

Instacart looks for in-place mutation discipline because their bag-scan code paths run inside hot shopper loops — avoid allocating new arrays.

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