2671

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:

• 1 <= number <= 105
• 1 <= frequency <= 105
• At most, 2 * 105 calls will be made to add, deleteOne, and hasFrequency in total.

Related Topics:
Hash Table, Design

Solution 1.

// OJ: https://leetcode.com/problems/frequency-tracker
// Author: github.com/lzl124631x
// Time: O(1) for all
// Space: O(N)
class FrequencyTracker {
    unordered_map<int, int> freq, cnt; // number -> frequency, frequency -> count of numbers with this 
frequency
public:
    FrequencyTracker() {
    }
    void add(int n) {
        int f = freq[n]++;
        cnt[f]--;
        cnt[f + 1]++;
    }
    void deleteOne(int n) {
        if (freq[n] == 0) return;
        int f = freq[n]--;
        cnt[f]--;
        cnt[f - 1]++;
    }
    bool hasFrequency(int f) {
        return cnt[f];
    }
};