# 2671. Frequency Tracker

## Description

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 occurence 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
[[], , , ]
Output
[null, null, null, true]

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



Example 2:

Input
[[], , , ]
Output
[null, null, null, false]

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



Example 3:

Input
[[], , , ]
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 
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.

## Solutions

• class FrequencyTracker {
private Map<Integer, Integer> cnt = new HashMap<>();
private Map<Integer, Integer> freq = new HashMap<>();

public FrequencyTracker() {
}

int f = cnt.getOrDefault(number, 0);
if (freq.getOrDefault(f, 0) > 0) {
freq.merge(f, -1, Integer::sum);
}
cnt.merge(number, 1, Integer::sum);
freq.merge(f + 1, 1, Integer::sum);
}

public void deleteOne(int number) {
int f = cnt.getOrDefault(number, 0);
if (f == 0) {
return;
}
freq.merge(f, -1, Integer::sum);
cnt.merge(number, -1, Integer::sum);
freq.merge(f - 1, 1, Integer::sum);
}

public boolean hasFrequency(int frequency) {
return freq.getOrDefault(frequency, 0) > 0;
}
}

/**
* Your FrequencyTracker object will be instantiated and called as such:
* FrequencyTracker obj = new FrequencyTracker();
* obj.deleteOne(number);
* boolean param_3 = obj.hasFrequency(frequency);
*/

• class FrequencyTracker {
public:
FrequencyTracker() {
}

int f = cnt[number];
if (f > 0) {
freq[f]--;
}
cnt[number]++;
freq[f + 1]++;
}

void deleteOne(int number) {
int f = cnt[number];
if (f == 0) {
return;
}
freq[f]--;
cnt[number]--;
freq[f - 1]++;
}

bool hasFrequency(int frequency) {
return freq[frequency] > 0;
}

private:
unordered_map<int, int> cnt;
unordered_map<int, int> freq;
};

/**
* Your FrequencyTracker object will be instantiated and called as such:
* FrequencyTracker* obj = new FrequencyTracker();
* obj->deleteOne(number);
* bool param_3 = obj->hasFrequency(frequency);
*/

• class FrequencyTracker:
def __init__(self):
self.cnt = defaultdict(int)
self.freq = defaultdict(int)

def add(self, number: int) -> None:
if self.freq[self.cnt[number]] > 0:
self.freq[self.cnt[number]] -= 1
self.cnt[number] += 1
self.freq[self.cnt[number]] += 1

def deleteOne(self, number: int) -> None:
if self.cnt[number] == 0:
return
self.freq[self.cnt[number]] -= 1
self.cnt[number] -= 1
self.freq[self.cnt[number]] += 1

def hasFrequency(self, frequency: int) -> bool:
return self.freq[frequency] > 0

# Your FrequencyTracker object will be instantiated and called as such:
# obj = FrequencyTracker()
# obj.deleteOne(number)
# param_3 = obj.hasFrequency(frequency)


• type FrequencyTracker struct {
cnt  map[int]int
freq map[int]int
}

func Constructor() FrequencyTracker {
return FrequencyTracker{map[int]int{}, map[int]int{} }
}

func (this *FrequencyTracker) Add(number int) {
f := this.cnt[number]
if f > 0 {
this.freq[f]--
}
this.cnt[number]++
this.freq[f+1]++
}

func (this *FrequencyTracker) DeleteOne(number int) {
f := this.cnt[number]
if f == 0 {
return
}
this.freq[f]--
this.cnt[number]--
this.freq[f-1]++
}

func (this *FrequencyTracker) HasFrequency(frequency int) bool {
return this.freq[frequency] > 0
}

/**
* Your FrequencyTracker object will be instantiated and called as such:
* obj := Constructor();
* obj.DeleteOne(number);
* param_3 := obj.HasFrequency(frequency);
*/

• class FrequencyTracker {
private cnt: Map<number, number>;
private freq: Map<number, number>;

constructor() {
this.cnt = new Map();
this.freq = new Map();
}

const f = this.cnt.get(number) || 0;
if (f > 0) {
this.freq.set(f, (this.freq.get(f) || 0) - 1);
}
this.cnt.set(number, f + 1);
this.freq.set(f + 1, (this.freq.get(f + 1) || 0) + 1);
}

deleteOne(number: number): void {
const f = this.cnt.get(number) || 0;
if (f === 0) {
return;
}
this.freq.set(f, (this.freq.get(f) || 0) - 1);
this.cnt.set(number, f - 1);
this.freq.set(f - 1, (this.freq.get(f - 1) || 0) + 1);
}

hasFrequency(frequency: number): boolean {
return (this.freq.get(frequency) || 0) > 0;
}
}

/**
* Your FrequencyTracker object will be instantiated and called as such:
* var obj = new FrequencyTracker()