2671 - Frequency Tracker\.

Medium

Design a data structure that keeps track of the values in it and answers some queries regarding their frequencies.

Implement the FrequencyTracker class.

• FrequencyTracker(): Initializes the FrequencyTracker object with an empty array initially.

• void add(int number): Adds number to the data structure.

• void deleteOne(int number): Deletes one occurrence of number from the data structure. The data structure may not contain number, and in this case nothing is deleted.

• bool hasFrequency(int frequency): Returns true if there is a number in the data structure that occurs frequency number of times, otherwise, it returns false.

Example 1:

Input "FrequencyTracker", "add", "add", "hasFrequency" [ [], 3, 3, 2]

Output: null, null, null, true

Explanation:

FrequencyTracker frequencyTracker = new FrequencyTracker();
frequencyTracker.add(3); // The data structure now contains [3]
frequencyTracker.add(3); // The data structure now contains [3, 3]
frequencyTracker.hasFrequency(2); // Returns true, because 3 occurs twice

Example 2:

Input "FrequencyTracker", "add", "deleteOne", "hasFrequency" [ [], 1, 1, 1]

Output: null, null, null, false

Explanation:

FrequencyTracker frequencyTracker = new FrequencyTracker();
frequencyTracker.add(1); // The data structure now contains [1]
frequencyTracker.deleteOne(1); // The data structure becomes empty []
frequencyTracker.hasFrequency(1); // Returns false, because the data structure is empty

Example 3:

Input "FrequencyTracker", "hasFrequency", "add", "hasFrequency" [ [], 2, 3, 1]

Output: null, false, null, true

Explanation:

FrequencyTracker frequencyTracker = new FrequencyTracker();
frequencyTracker.hasFrequency(2); // Returns false, because the data structure is empty
frequencyTracker.add(3); // The data structure now contains [3]
frequencyTracker.hasFrequency(1); // Returns true, because 3 occurs once

Constraints:

• <code>1 <= number <= 10⁵</code>

• <code>1 <= frequency <= 10⁵</code>

• At most, <code>2 * 10⁵</code> calls will be made to add, deleteOne, and hasFrequency in total.