First Value Appearing Exactly Once in an Append-Only Queue

### Problem Implement the native FirstUnique class. The constructor initializes the queue with nums. add appends one value. showFirstUnique returns the earliest queue value whose total occurrence count is exactly one,or -1 if none exists. Appending never removes an existing queue occurrence. ```cpp class FirstUnique { public: FirstUnique(vector<int>& nums); int showFirstUnique(); void add(int value); }; ``` ### Examples **Example 1:** ```text Calls: FirstUnique([4,6,4,9]); showFirstUnique(); add(6); showFirstUnique(); add(9); showFirstUnique() Returned query values: [6,9,-1] Explanation: The initially unique values are6 then9; duplicating each removes its eligibility. ``` **Example 2:** ```text Calls: FirstUnique([8,8]); showFirstUnique(); add(5); showFirstUnique(); add(8); showFirstUnique() Returned query values: [-1,5,5] Explanation: The already repeated8 never becomes unique again, while5 remains unique. ``` ### Constraints - nums has length 1 to 100000. - Each nums value and added value lies in `[1,100000000]`. - At most 50000 calls in total are made to showFirstUnique and add. ### Contract considerations Use the constructor, `add`, and `showFirstUnique` methods shown above; successive calls share the queue's accumulated history. ```hint Read the boundary condition Check successive queries as values are appended, including repeated initial values and a history with no unique value. Earlier occurrences remain in the history. ``` ### Boundary reminders Use the constructor, `add`, and `showFirstUnique` methods shown above; successive calls share the queue's accumulated history.