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 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.

Solutions

Solution 1: Hash Table

We define two hash tables, where $cnt$ is used to record the occurrence count of each number, and $freq$ is used to record the count of numbers with each frequency.

For the add operation, we directly decrement the value corresponding to $cnt[number]$ in the hash table $freq$, then increment $cnt[number]$, and finally increment the value corresponding to $cnt[number]$ in $freq$.

For the deleteOne operation, we first check if $cnt[number]$ is greater than zero. If it is, we decrement the value corresponding to $cnt[number]$ in the hash table $freq$, then decrement $cnt[number]$, and finally increment the value corresponding to $cnt[number]$ in $freq$.

For the hasFrequency operation, we directly return whether $freq[frequency]$ is greater than zero.

In terms of time complexity, since we use hash tables, the time complexity of each operation is $O(1)$. The space complexity is $O(n)$, where $n$ is the number of distinct numbers.

Python3

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

    def add(self, number: int) -> None:
        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]:
            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.add(number)
# obj.deleteOne(number)
# param_3 = obj.hasFrequency(frequency)

Java

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

    public FrequencyTracker() {
    }

    public void add(int number) {
        freq.merge(cnt.getOrDefault(number, 0), -1, Integer::sum);
        cnt.merge(number, 1, Integer::sum);
        freq.merge(cnt.get(number), 1, Integer::sum);
    }

    public void deleteOne(int number) {
        if (cnt.getOrDefault(number, 0) > 0) {
            freq.merge(cnt.get(number), -1, Integer::sum);
            cnt.merge(number, -1, Integer::sum);
            freq.merge(cnt.get(number), 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.add(number);
 * obj.deleteOne(number);
 * boolean param_3 = obj.hasFrequency(frequency);
 */

C++

class FrequencyTracker {
public:
    FrequencyTracker() {
    }

    void add(int number) {
        freq[cnt[number]]--;
        cnt[number]++;
        freq[cnt[number]]++;
    }

    void deleteOne(int number) {
        if (cnt[number]) {
            freq[cnt[number]]--;
            cnt[number]--;
            freq[cnt[number]]++;
        }
    }

    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->add(number);
 * obj->deleteOne(number);
 * bool param_3 = obj->hasFrequency(frequency);
 */

Go

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) {
	this.freq[this.cnt[number]]--
	this.cnt[number]++
	this.freq[this.cnt[number]]++
}

func (this *FrequencyTracker) DeleteOne(number int) {
	if this.cnt[number] > 0 {
		this.freq[this.cnt[number]]--
		this.cnt[number]--
		this.freq[this.cnt[number]]++
	}
}

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.Add(number);
 * obj.DeleteOne(number);
 * param_3 := obj.HasFrequency(frequency);
 */

TypeScript

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

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

    add(number: number): void {
        const f = this.cnt.get(number) || 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) {
            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()
 * obj.add(number)
 * obj.deleteOne(number)
 * var param_3 = obj.hasFrequency(frequency)
 */

Rust

use std::collections::HashMap;

struct FrequencyTracker {
    cnt: HashMap<i32, i32>,
    freq: HashMap<i32, i32>,
}

/**
 * `&self` means the method takes an immutable reference.
 * If you need a mutable reference, change it to `&mut self` instead.
 */
impl FrequencyTracker {
    fn new() -> Self {
        FrequencyTracker {
            cnt: HashMap::new(),
            freq: HashMap::new(),
        }
    }

    fn add(&mut self, number: i32) {
        let count = self.cnt.entry(number).or_insert(0);
        let prev_freq = self.freq.entry(*count).or_insert(0);
        *prev_freq -= 1;
        *count += 1;
        let new_freq = self.freq.entry(*count).or_insert(0);
        *new_freq += 1;
    }

    fn delete_one(&mut self, number: i32) {
        if let Some(count) = self.cnt.get_mut(&number) {
            if *count > 0 {
                if let Some(prev_freq) = self.freq.get_mut(count) {
                    *prev_freq -= 1;
                }
                *count -= 1;
                if let Some(new_freq) = self.freq.get_mut(count) {
                    *new_freq += 1;
                }
            }
        }
    }

    fn has_frequency(&self, frequency: i32) -> bool {
        self.freq.get(&frequency).map_or(false, |&freq| freq > 0)
    }
}