In many real$-$world applications, such as network data monitoring or sales

tracking, identifying unique events or items dynamically as data continuously

arrives is crucial. This assignment focuses on designing and implementing a

data structure that efficiently tracks the first unique number in a stream of

non$-$negative integers. Your task is to implement a system capable of adding

numbers to the stream and identifying the first unique number at any moment.

Problem Statement

Your implementation should support two primary operations:

$-$ add$($number$)$: Adds a number to the data stream.

$-$ showFirstUnique$()$: Returns the first number that appears only once in the

data stream. If there is no such number, return $-1.$

Key Requirements

Utilize a Hashmap to track the counts and positions of each number within a

linked list, efficiently managing duplicates and their occurrences.

Implement a Doubly Linked List to maintain numbers that currently have a

unique occurrence, preserving the order they were added.

Aim for O$(1)$ average time complexity for both add and showFirstUnique

operations, ensuring optimal performance as the data stream grows.

Implement the add and showFirstUnique methods as specified.

Ensure your code is functional, as non$-$functional code will result in a zero

score.