lru-cache.md

LRU Cache

Design a data structure that follows the constraints of a Least Recently Used (LRU) cache.

Implement the LRUCache class:

• LRUCache(int capacity) Initialize the LRU cache with positive size capacity.
• int get(int key) Return the value of the key if the key exists, otherwise return -1.
• void put(int key, int value) Update the value of the key if the key exists. Otherwise, add the key-value pair to the cache. If the number of keys exceeds the capacity from this operation, evict the least recently used key. The functions get and put must each run in O(1) average time complexity.

Practice Link

C++ Implementation

class Node{
public:
    int value;
    int key;
    Node* next;
    Node* prev;

    Node(int key, int value)
    {
        this->key = key;
        this->value = value;
    }
};


class LRUCache {
    Node* head = new Node(-1,-1);
    Node* tail = new Node(-1,-1);
    unordered_map<int,Node*> mp;
    int capacity;

    void deleteNode(Node* curr)
    {
        curr->prev->next = curr->next;
        curr->next->prev = curr->prev;
        delete(curr);
    }

    void addNode(Node* curr)
    {
        curr->next = head->next;
        curr->prev = head;

        head->next = curr;
        curr->next->prev = curr;
    }

public:
    LRUCache(int capacity) {
        this->capacity = capacity;
        head->next = tail;
        tail->prev = head;
    }
    
    int get(int key) {
        if(mp.find(key) == mp.end())
            return -1;
        Node* curr = mp[key];
        int result = curr->value;

        mp.erase(key);
        deleteNode(curr);

        addNode(new Node(key, result));
        mp[key] = head->next;

        return result;
    }
    
    void put(int key, int value) {

        if(mp.find(key) != mp.end())
        {
            Node* curr = mp[key];
            mp.erase(key);
            deleteNode(curr);
        }

        if(mp.size() == this->capacity)
        {
            mp.erase(tail->prev->key);
            deleteNode(tail->prev);
        }
        addNode(new Node(key,value));
        mp[key] = head->next;
    }
};

/**
 * Your LRUCache object will be instantiated and called as such:
 * LRUCache* obj = new LRUCache(capacity);
 * int param_1 = obj->get(key);
 * obj->put(key,value);
 */

JAVA Implementation

class Node{
    int key;
    int val;
    Node prev;
    Node next;

    public Node(int key, int val)
    {
        this.key = key;
        this.val = val;
        this.prev = null;
        this.next = null;
    }
}
class LRUCache {
    private int capacity;
    private Map<Integer, Node> cache;
    private Node latest;
    private Node oldest;

    private void addNode(Node curr)
    {
        curr.next = latest.next;
        curr.prev = latest;

        latest.next = curr;
        curr.next.prev = curr;
    }

    private void detachNode(Node curr)
    {
        curr.prev.next = curr.next;
        curr.next.prev = curr.prev;
    }


    public LRUCache(int capacity) {
        this.capacity = capacity;
        this.cache = new HashMap<>();
        this.latest = new Node(-1,-1);
        this.oldest = new Node(-1,-1);
        latest.next = oldest;
        oldest.prev = latest;
    }
    
    public int get(int key) {
        if(!cache.containsKey(key))
            return -1;

        Node curr = cache.get(key);
        int result = curr.val;

        detachNode(curr);

        addNode(curr);
        cache.put(key,curr);

        return result;
    }
    
    public void put(int key, int value) {
        if(cache.containsKey(key))
        {
            Node curr = cache.get(key);
            cache.remove(key);
            detachNode(curr);
        }
        if(cache.size()==capacity)
        {
            cache.remove(oldest.prev.key);
            detachNode(oldest.prev);
        }

        Node curr = new Node(key, value);
        addNode(curr);
        cache.put(key, curr);
    }
}

/**
 * Your LRUCache object will be instantiated and called as such:
 * LRUCache obj = new LRUCache(capacity);
 * int param_1 = obj.get(key);
 * obj.put(key,value);
 */