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4. Remove Duplicates from Sorted Array

easyAsked at Slack

In-place dedupe a sorted array and return the new length.

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. Return the number of unique elements; the first k elements of nums must hold the unique values.

Constraints

  • 1 <= nums.length <= 3*10^4
  • nums 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. Set + rewrite

Copy to set, write back.

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

Tradeoff:

2. Two pointers

Slow pointer marks the next unique slot; fast scans. Write when fast value differs from slow's value.

Time
O(n)
Space
O(1)
function removeDuplicates(nums) {
  if (nums.length === 0) 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:

Slack-specific tips

Slack values in-place mutations for memory-bound mobile clients — show you understand the cost of extra allocations.

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