Remove Duplicates from Sorted Array II

Remove Duplicates from Sorted Array II is a Medium Data Structures and Algorithms interview problem you can solve, run and submit on Thita.ai. It belongs to the Two Pointers pattern, in the In-place Array Modification subpattern.

Problem statement

Given an integer array nums sorted in non-decreasing order, remove some duplicates in-place such that each unique element appears at most twice. The relative order of the elements should be kept the same.

Since it is impossible to change the length of the array in some languages, you must instead have the result be placed in the first part of the array nums. More formally, if there are k elements after removing the duplicates, then the first k elements of nums should hold the final result. It does not matter what you leave beyond the first k elements.

Return k after placing the final result in the first k slots of nums.

Do not allocate extra space for another array. You must do this by modifying the input array in-place with O(1) extra memory.

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

Example 2:

Input: nums = [0,0,1,1,1,1,2,3,3] Output: 7, nums = [0,0,1,1,2,3,3,,] Explanation: Your function should return k = 7, with the first seven elements of nums being 0, 0, 1, 1, 2, 3 and 3 respectively. It does not matter what you leave beyond the returned k (hence they are underscores).

Constraints:

- 1 <= nums.length <= 3 104

- -104 <= nums[i] <= 104

- nums is sorted in non-decreasing order.

Topics and companies

Topics: Array, Two Pointers.

Reported in interviews at Microsoft.

How to practise Remove Duplicates from Sorted Array II on Thita.ai

Starter code is provided in C, C#, C++, Go, Java, JavaScript and Python. Your solution runs against 5 test cases for this problem, with the AI coach available for a hint when you are stuck rather than a finished answer. The reference solution runs in O(n), where n is the length of nums. Each element is read and written at most once. time and O(1), since the operation is done in-place and only a few integer variables are used. space.

Open Remove Duplicates from Sorted Array II in the code editor, or read the Remove Duplicates from Sorted Array II editorial for a worked solution with its approach and complexity analysis.

Where Remove Duplicates from Sorted Array II sits in the DSA pattern sheet

Problems related to Remove Duplicates from Sorted Array II

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