Grokking the Coding Interview: Patterns for Coding Questions
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Solution: Find Non-Duplicate Number Instances

## Problem Statement

Given an array of sorted numbers, move all non-duplicate number instances at the beginning of the array in-place. The relative order of the elements should be kept the same and you should not use any extra space so that the solution has constant space complexity i.e., O(1).

Move all the unique number instances at the beginning of the array and after moving return the length of the subarray that has no duplicate in it.

Example 1:

``````Input: [2, 3, 3, 3, 6, 9, 9]
Output: 4
Explanation: The first four elements after moving element will be [2, 3, 6, 9].
``````

Example 2:

``````Input: [2, 2, 2, 11]
Output: 2
Explanation: The first two elements after moving elements will be [2, 11].
``````

Constraints:

• 1 <= nums.length <= 3 * 10<sup>4</sup>
• `-100 <= nums[i] <= 100`
• `nums` is sorted in non-decreasing order.

## Solution

In this problem, we need to separate the duplicates in-place such that the resultant length of the array remains sorted. As the input array is sorted, therefore, one way to do this is to shift the elements left whenever we encounter duplicates. In other words, we will keep one pointer for iterating the array and one pointer for placing the next non-duplicate number. So our algorithm will be to iterate the array and whenever we see a non-duplicate number we move it next to the last non-duplicate number we’ve seen.

Here is the visual representation of this algorithm for Example-1:

## Code

Here is what our algorithm will look like:

Python3
Python3

. . .
You must run your code first

Time Complexity

The time complexity of the above algorithm will be O(N), where ‘N’ is the total number of elements in the given array.

Space Complexity

The algorithm runs in constant space O(1).

## Similar Questions

Problem 1: Given an unsorted array of numbers and a target ‘key’, remove all instances of ‘key’ in-place and return the new length of the array.

Example 1:

``````Input: [3, 2, 3, 6, 3, 10, 9, 3], Key=3
Output: 4
Explanation: The first four elements after removing every 'Key' will be [2, 6, 10, 9].
``````

Example 2:

``````Input: [2, 11, 2, 2, 1], Key=2
Output: 2
Explanation: The first two elements after removing every 'Key' will be [11, 1].
``````

### Solution:

This problem is quite similar to our parent problem. We can follow a two-pointer approach and shift numbers left upon encountering the ‘key’. Here is what the code will look like:

Python3
Python3

. . .
You must run your code first

Time and Space Complexity: The time complexity of the above algorithm will be O(N), where ‘N’ is the total number of elements in the given array.

The algorithm runs in constant space O(1).

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