LFU Cache Implementation – LeetCode

Design and implement a data structure for a Least Frequently Used (LFU) cache.

Implement the LFUCache class:

• LFUCache(int capacity) Initializes the object with the capacity of the data structure.
• int get(int key) Gets the value of the key if the key exists in the cache. Otherwise, returns -1.
• void put(int key, int value) Update the value of the key if present, or inserts the key if not already present. When the cache reaches its capacity, it should invalidate and remove the least frequently used key before inserting a new item. For this problem, when there is a tie (i.e., two or more keys with the same frequency), the least recently used key would be invalidated.

Example 1:

Input
["LFUCache", "put", "put", "get", "put", "get", "get", "put", "get", "get", "get"]
[[2], [1, 1], [2, 2], [1], [3, 3], [2], [3], [4, 4], [1], [3], [4]]
Output
[null, null, null, 1, null, -1, 3, null, -1, 3, 4]

This we are going to implement using a combination of a hash table (unordered_map) to store the key-value pairs, the frequency of each key, and a list to maintain the order of the keys based on their frequency. The put function adds a new key-value pair to the cache and removes the least frequently used key if the capacity is exceeded. The get function retrieves the value of a key and updates its frequency. The update_freq function updates the frequency of a key and rearranges it in the list accordingly.

Below is the implementation:

#include <unordered_map>
#include <list>

class LFUCache {
public:
    int capacity;
    std::unordered_map<int, std::pair<int, int>> key_val_freq;
    std::unordered_map<int, std::list<int>::iterator> key_iter;
    std::list<int> freq_list;

    LFUCache(int _capacity) : capacity(_capacity) {}

    int get(int key) {
        if (key_val_freq.find(key) == key_val_freq.end()) return -1;
        update_freq(key);
        return key_val_freq[key].first;
    }

    void put(int key, int value) {
        if (capacity <= 0) return;
        if (key_val_freq.find(key) != key_val_freq.end()) {
            update_freq(key);
            key_val_freq[key].first = value;
            return;
        }
        if (key_val_freq.size() == capacity) {
            int k = freq_list.back();
            freq_list.pop_back();
            key_val_freq.erase(k);
            key_iter.erase(k);
        }
        freq_list.push_front(key);
        key_iter[key] = freq_list.begin();
        key_val_freq[key] = { value, 1 };
    }

private:
    void update_freq(int key) {
        auto it = key_iter[key];
        int freq = key_val_freq[key].second;
        key_val_freq[key].second++;
        freq_list.erase(it);
        for (auto i = freq_list.begin(); i != freq_list.end(); i++) {
            if (key_val_freq[*i].second > freq) {
                freq_list.insert(i, key);
                key_iter[key] = --i;
                return;
            }
        }
        freq_list.push_back(key);
        key_iter[key] = --freq_list.end();
    }
};