LeetCode #26: Remove Duplicates from Sorted Array — Solved in Java

Simple Pass with a Small Buffer and Two Pointers In-Place

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LeetCode’s Remove Duplicates from Sorted Array problem gives you a sorted list of numbers and asks you to remove all repeated values directly inside the same array. The idea is to have each unique number appear only once at the front and return how many unique values remain.

Thinking about how to solve this in Java means paying attention to how array positions are managed. Every element can be replaced but never truly removed, so the focus shifts toward rearranging rather than deleting. The right mindset is to track how numbers move through the array while keeping duplicates from being written forward. That perspective helps make sense of how the two-pointer pattern works and why it’s a common technique in array manipulation problems.

LeetCode: Remove Duplicates from Sorted Array

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.

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 * 10^4

• -100 <= nums[i] <= 100

• nums is sorted in non-decreasing order.

Solution 1: Simple Pass with a Small Buffer

This version collects unique values into a temporary list before placing them back into the original array and returning the new length. It walks through the numbers one by one, keeping only the first time each value appears and skipping the rest. The separation between collecting and rewriting makes it easier to follow the logic and see how duplicates get filtered out step by step.