Coding

Remove duplicates from sorted array

Asked at

Ninjacart

Twilio

1Give yourself 5 minutes

2Answer out loud, not in your head

3Then compare with the answer below

The problem

Given an integer array nums sorted in non-decreasing order, remove all duplicates in-place so that each unique element appears only once.

Return the number of unique elements in the array.

If the number of unique elements be** k,** then,

Change the array nums** such that the first k **elements of nums contain the unique values in the order that they were present originally.
The remaining elements, as well as the size of the array does not matter in terms of correctness.
The driver code will assess correctness by printing and checking only the first k elements of the modified array.

An array sorted in **non-decreasing order is an array where every element to the right of an element is either equal to or greater **in value than that element.

Input: nums = [0, 0, 3, 3, 5, 6] Output: 4 Explanation: Resulting array = [0, 3, 5, 6, _, _] There are 4 distinct elements in nums and the elements marked as _ can have any value.

Input: nums = [-2, 2, 4, 4, 4, 4, 5, 5] Output: 4 Explanation: Resulting array = [-2, 2, 4, 5, _, _, _, _] There are 4 distinct elements in nums and the elements marked as _ can have any value.

Input: nums = [-30, -30, 0, 0, 10, 20, 30, 30] Select the possible resulting array.

1 <= nums.length <= 105
-104 <= nums[i] <= 104
nums is sorted in non-decreasing order.

cpp

class Solution {
public:
    int removeDuplicates(vector<int>& nums) {
        
    }
};

java

class Solution {
    public int removeDuplicates(int[] nums) {
        
    }
}

python

class Solution:
    def removeDuplicates(self, nums: list[int]) -> int:

javascript

class Solution {
    removeDuplicates(nums) {

    }
}

csharp

public class Solution {
    public int removeDuplicates(List<int> nums) {
        
    }
}

func removeDuplicates(nums []int) int {
    //your code goes here
}

Stuck? Show a way to structure it+

01State that the input is sorted and mutation is allowed
02Initialize the write pointer after the first item
03Scan with a read pointer and compare against the last kept item
04Return the write length

Reference answer

Start by restating the input contract for Remove duplicates from sorted array and testing the smallest boundary case before choosing the algorithm. Use a write pointer that represents the next unique position. Scan each value once; whenever it differs from the last kept value, write it and advance the pointer. The prefix before the write pointer is the deduplicated in-place answer. Explain the invariant while coding, then show how each update preserves it. The two-pointer scan is O(n) time and O(1) auxiliary space because it rewrites the array in place. Walk through one nontrivial example aloud so the interviewer can see why the method reaches the intended result. Finally, call out input assumptions and failure cases rather than silently relying on them. A clean implementation should keep state local, use descriptive names, and avoid work that the invariant makes unnecessary.

Then expect these follow-ups

How would you allow at most two copies?
Tests: follow-up reasoning
What changes if the array is unsorted?
Tests: constraint adaptation

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