LeetCode: 26. Remove Duplicates from Sorted Array

The Problem

Given an integer array nums sorted in non-decreasing order, remove the duplicates in-place such that each unique element appears only once. The relative order of the elements should be kept the same. Then return the number of unique elements in nums.

Consider the number of unique elements of nums to be k, to get accepted, you need to do the following things:

• Change the array nums such that the first k elements of nums contain the unique elements in the order they were present in nums initially. The remaining elements of nums are not important as well as the size of nums.
• Return k.

Custom Judge:

The judge will test your solution with the following code:

int[] nums = [...]; // Input array
int[] expectedNums = [...]; // The expected answer with correct length

int k = removeDuplicates(nums); // Calls your implementation

assert k == expectedNums.length;
for (int i = 0; i < k; i++) {
    assert nums[i] == expectedNums[i];
}

If all assertions pass, then your solution will be accepted.

Examples

Example 1:

Input: nums = [1,1,2]
Output: 2, nums = [1,2,_]
Explanation: Your function should return k = 2, with the first two elements of nums being 1 and 2 respectively.
It does not matter what you leave beyond the returned k (hence they are underscores).
Example 2:

Input: nums = [0,0,1,1,1,2,2,3,3,4]
Output: 5, nums = [0,1,2,3,4,_,_,_,_,_]
Explanation: Your function should return k = 5, with the first five elements of nums being 0, 1, 2, 3, and 4 respectively.
It does not matter what you leave beyond the returned k (hence they are underscores).

Constraints:

• 1 <= nums.length <= 3 * 104
• -100 <= nums[i] <= 100
• nums is sorted in non-decreasing order.

The Solution

We opted for a simple approach using a for loop

int removeDuplicates(vector<int>& nums) {
	int lastNonDuplicate = 1;

	for(int i = 1; i < nums.size(); ++i){
		lastNonDuplicate += ( nums[i] != nums[i - 1]);
		nums[lastNonDuplicate - 1] = nums[i];
	}

	return lastNonDuplicate;
}

Explanation:

• The function removeDuplicates takes a reference to a sorted vector of integers as its parameter.
• It iterates through the array, keeping track of the last index where a non-duplicate element should be placed.
• If the current element is different from the previous one, it updates the lastNonDuplicate index.
• It overwrites duplicates with non-duplicate elements in-place.
• Finally, it returns the count of unique elements.

This approach achieves removal of duplicates in linear time complexity, making it efficient for large arrays.