Remove Duplicates from Sorted Array Easy 0 attempts
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Remove Duplicates from Sorted Array

Easy ArrayTwo Pointers LeetCode

Given an integer array nums sorted in non-decreasing order, remove the duplicates in-place so that each unique element appears only once. Return the count of unique elements k. The first k elements of nums should hold the unique values in their original order. What you leave beyond index k doesn't matter.

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,_,_,_,_,_]

Edge cases: Array with one element. All elements are the same.

Sample Input
nums = [1,1,2]
Sample Output
2
Constraints
  • 1 <= nums.length <= 3 * 10^4
  • -100 <= nums[i] <= 100
  • nums is sorted in non-decreasing order
Test Cases
Case 1
Args: [[1,1,2]] Expected: 2
Topics
ArrayTwo Pointers

Approach: Two Pointers

Since the array is sorted, duplicates are always adjacent. Keep a slow pointer k that marks where the next unique value should go. Walk a fast pointer through the array — whenever you see a value different from nums[k-1], write it at position k and advance k.

function removeDuplicates(nums) {
  if (nums.length === 0) return 0;
  let k = 1;

  for (let i = 1; i < nums.length; i++) {
    if (nums[i] !== nums[k - 1]) {
      nums[k] = nums[i];
      k++;
    }
  }
  return k;
}

Time Complexity: O(n)

Space Complexity: O(1)

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