146. LRU Cache – Leetcode Solutions

Example 1:

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

Explanation
LRUCache lRUCache = new LRUCache(2);
lRUCache.put(1, 1); // cache is {1=1}
lRUCache.put(2, 2); // cache is {1=1, 2=2}
lRUCache.get(1);    // return 1
lRUCache.put(3, 3); // LRU key was 2, evicts key 2, cache is {1=1, 3=3}
lRUCache.get(2);    // returns -1 (not found)
lRUCache.put(4, 4); // LRU key was 1, evicts key 1, cache is {4=4, 3=3}
lRUCache.get(1);    // return -1 (not found)
lRUCache.get(3);    // return 3
lRUCache.get(4);    // return 4

from collections import OrderedDict

class LRUCache:
    def __init__(self, capacity: int):
        """
        Initialize the LRU cache with positive size capacity.
        """
        self.cache = OrderedDict()
        self.capacity = capacity

    def get(self, key: int) -> int:
        """
        Return the value of the key if the key exists, otherwise return -1.
        """
        if key not in self.cache:
            return -1
        
        # Move the accessed key to the end to mark it as recently used
        self.cache.move_to_end(key)
        return self.cache[key]

    def put(self, key: int, value: int) -> None:
        """
        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.
        """
        if key in self.cache:
            # Update the value and move the key to the end to mark it as recently used
            self.cache.move_to_end(key)
        self.cache[key] = value
        
        if len(self.cache) > self.capacity:
            # Pop the first item (least recently used) if the capacity is exceeded
            self.cache.popitem(last=False)

/**
 * LRUCache class that simulates an LRU (Least Recently Used) cache.
 * @param {number} capacity - The maximum number of items the cache can hold.
 */
var LRUCache = function(capacity) {
    this.capacity = capacity;
    this.cache = new Map();
};

/** 
 * Gets the value of the key if the key exists in the cache.
 * Moves the accessed key to the end to mark it as recently used.
 * @param {number} key
 * @return {number} - The value of the key, or -1 if the key does not exist.
 */
LRUCache.prototype.get = function(key) {
    if (!this.cache.has(key)) {
        return -1; // Key does not exist
    }
    const value = this.cache.get(key);
    // Move the key to the end to mark it as recently used
    this.cache.delete(key);
    this.cache.set(key, value);
    return value;
};

/** 
 * Updates the value of the key if the key exists. Otherwise, adds the key-value pair to the cache.
 * If the number of keys exceeds the capacity, evicts the least recently used key.
 * @param {number} key 
 * @param {number} value
 * @return {void}
 */
LRUCache.prototype.put = function(key, value) {
    if (this.cache.has(key)) {
        // If the key already exists, delete it so we can re-insert it as the most recently used
        this.cache.delete(key);
    }
    this.cache.set(key, value);
    
    // If the cache exceeds the capacity, remove the least recently used item
    if (this.cache.size > this.capacity) {
        // keys().next().value retrieves the first item's key in the Map
        const oldestKey = this.cache.keys().next().value;
        this.cache.delete(oldestKey);
    }
};

import java.util.HashMap;

class LRUCache {

    // Node class to represent each entry in the cache
    private class Node {
        int key;
        int value;
        Node prev;
        Node next;

        public Node(int key, int value) {
            this.key = key;
            this.value = value;
        }
    }

    private int capacity; // Maximum capacity of the cache
    private HashMap<Integer, Node> map; // HashMap to store the keys and nodes
    private Node head; // Dummy head of the doubly linked list
    private Node tail; // Dummy tail of the doubly linked list

    public LRUCache(int capacity) {
        this.capacity = capacity;
        this.map = new HashMap<>();
        this.head = new Node(-1, -1); // Initialize dummy head
        this.tail = new Node(-1, -1); // Initialize dummy tail
        head.next = tail;
        tail.prev = head;
    }
    
    public int get(int key) {
        if (!map.containsKey(key)) {
            return -1; // Return -1 if the key is not found
        }
        Node node = map.get(key);
        remove(node); // Remove the node from its current position
        insertToHead(node); // Insert the node to the head (most recently used position)
        return node.value;
    }
    
    public void put(int key, int value) {
        if (map.containsKey(key)) {
            // If the key exists, update the value and move it to the head
            Node node = map.get(key);
            node.value = value;
            remove(node);
            insertToHead(node);
        } else {
            // If the key does not exist, create a new node
            Node newNode = new Node(key, value);
            if (map.size() >= capacity) {
                // If the cache is full, remove the least recently used item
                Node lru = tail.prev;
                remove(lru);
                map.remove(lru.key);
            }
            // Add the new node to the cache and insert it to the head
            map.put(key, newNode);
            insertToHead(newNode);
        }
    }

    // Helper method to remove a node from the doubly linked list
    private void remove(Node node) {
        node.prev.next = node.next;
        node.next.prev = node.prev;
    }

    // Helper method to insert a node to the head of the doubly linked list
    private void insertToHead(Node node) {
        node.next = head.next;
        node.prev = head;
        head.next.prev = node;
        head.next = node;
    }
}

using System;
using System.Collections.Generic;

public class LRUCache {

    // Node class to represent each entry in the cache
    private class Node {
        public int Key;
        public int Value;
        public Node Prev;
        public Node Next;
        
        public Node(int key, int value) {
            this.Key = key;
            this.Value = value;
        }
    }

    private int capacity; // Maximum capacity of the cache
    private Dictionary<int, Node> cache; // Dictionary to store key-node pairs
    private Node head; // Dummy head of the doubly linked list
    private Node tail; // Dummy tail of the doubly linked list

    public LRUCache(int capacity) {
        this.capacity = capacity;
        this.cache = new Dictionary<int, Node>();
        this.head = new Node(0, 0); // Initialize dummy head
        this.tail = new Node(0, 0); // Initialize dummy tail
        head.Next = tail;
        tail.Prev = head;
    }

    public int Get(int key) {
        if (!cache.ContainsKey(key)) {
            return -1; // Return -1 if the key is not found
        }
        Node node = cache[key];
        Remove(node); // Remove the node from its current position
        InsertToHead(node); // Insert the node to the head (most recently used position)
        return node.Value;
    }

    public void Put(int key, int value) {
        if (cache.ContainsKey(key)) {
            // If the key exists, update the value and move it to the head
            Node node = cache[key];
            node.Value = value;
            Remove(node);
            InsertToHead(node);
        } else {
            // If the key does not exist, create a new node
            Node newNode = new Node(key, value);
            if (cache.Count >= capacity) {
                // If the cache is full, remove the least recently used item
                Node lru = tail.Prev;
                Remove(lru);
                cache.Remove(lru.Key);
            }
            // Add the new node to the cache and insert it to the head
            cache[key] = newNode;
            InsertToHead(newNode);
        }
    }

    // Helper method to remove a node from the doubly linked list
    private void Remove(Node node) {
        node.Prev.Next = node.Next;
        node.Next.Prev = node.Prev;
    }

    // Helper method to insert a node to the head of the doubly linked list
    private void InsertToHead(Node node) {
        node.Next = head.Next;
        node.Prev = head;
        head.Next.Prev = node;
        head.Next = node;
    }
}