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4. Remove Duplicates from Sorted Array

easyAsked at Swiggy

Compact a sorted array in-place so each value appears once, returning the new length.

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

Problem

Given a sorted integer array nums, remove duplicates in-place so that each unique element appears only once. Return the count k of unique elements. The first k positions of nums must hold those values in order.

Constraints

  • 1 <= nums.length <= 3 * 10^4
  • Array is sorted non-decreasing
  • Must modify in-place with O(1) extra memory

Examples

Example 1

Input
nums=[1,1,2]
Output
k=2, nums=[1,2,_]

Example 2

Input
nums=[0,0,1,1,1,2,2,3,3,4]
Output
k=5, nums=[0,1,2,3,4,_,_,_,_,_]

Approaches

1. Set copy back

Dedup via Set 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

Write pointer advances only when current value differs from the last written value. Read pointer scans every element.

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[write - 1]) {
      nums[write] = nums[read];
      write++;
    }
  }
  return write;
}

Tradeoff:

Swiggy-specific tips

Swiggy uses this warm-up to test in-place mutation discipline before moving into courier-route compaction problems where memory pressure matters on edge devices.

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Output

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