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4. Remove Duplicates from Sorted Array

easyAsked at Flipkart

Strip duplicates in-place from a sorted array — Flipkart uses this to test two-pointer fluency before scaling to deduping customer review streams.

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

Problem

Given an integer array nums sorted in non-decreasing order, remove duplicates in-place so each unique element appears once. Return the count k of unique elements; the first k slots must hold the unique values.

Constraints

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

Insert into a set, then copy back.

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

Tradeoff:

2. Two pointers in-place

Write pointer trails read pointer; only copy on a new distinct value.

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:

Flipkart-specific tips

Flipkart loves to see an in-place solution justified with memory-pressure reasoning — their order-batch jobs run on tight-RAM workers.

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