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146. LRU Cache

Description

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.

 

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

 

Constraints:

  • 1 <= capacity <= 3000
  • 0 <= key <= 104
  • 0 <= value <= 105
  • At most 2 * 105 calls will be made to get and put.

Solutions

Solution 1: Hash Table + Doubly Linked List

We can implement an LRU (Least Recently Used) cache using a "hash table" and a "doubly linked list".

  • Hash Table: Used to store the key and its corresponding node location.
  • Doubly Linked List: Used to store node data, sorted by access time.

When accessing a node, if the node exists, we delete it from its original position and reinsert it at the head of the list. This ensures that the node stored at the tail of the list is the least recently used node. When the number of nodes exceeds the maximum cache space, we eliminate the node at the tail of the list.

When inserting a node, if the node exists, we delete it from its original position and reinsert it at the head of the list. If it does not exist, we first check if the cache is full. If it is full, we delete the node at the tail of the list and insert the new node at the head of the list.

The time complexity is $O(1)$, and the space complexity is $O(\textit{capacity})$.

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class Node:
    def __init__(self, key=0, val=0):
        self.key = key
        self.val = val
        self.prev = None
        self.next = None


class LRUCache:
    def __init__(self, capacity: int):
        self.cache = {}
        self.head = Node()
        self.tail = Node()
        self.capacity = capacity
        self.size = 0
        self.head.next = self.tail
        self.tail.prev = self.head

    def get(self, key: int) -> int:
        if key not in self.cache:
            return -1
        node = self.cache[key]
        self.move_to_head(node)
        return node.val

    def put(self, key: int, value: int) -> None:
        if key in self.cache:
            node = self.cache[key]
            node.val = value
            self.move_to_head(node)
        else:
            node = Node(key, value)
            self.cache[key] = node
            self.add_to_head(node)
            self.size += 1
            if self.size > self.capacity:
                node = self.remove_tail()
                self.cache.pop(node.key)
                self.size -= 1

    def move_to_head(self, node):
        self.remove_node(node)
        self.add_to_head(node)

    def remove_node(self, node):
        node.prev.next = node.next
        node.next.prev = node.prev

    def add_to_head(self, node):
        node.next = self.head.next
        node.prev = self.head
        self.head.next = node
        node.next.prev = node

    def remove_tail(self):
        node = self.tail.prev
        self.remove_node(node)
        return node


# Your LRUCache object will be instantiated and called as such:
# obj = LRUCache(capacity)
# param_1 = obj.get(key)
# obj.put(key,value)

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

    Node() {
    }

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

class LRUCache {
    private Map<Integer, Node> cache = new HashMap<>();
    private Node head = new Node();
    private Node tail = new Node();
    private int capacity;
    private int size;

    public LRUCache(int capacity) {
        this.capacity = capacity;
        head.next = tail;
        tail.prev = head;
    }

    public int get(int key) {
        if (!cache.containsKey(key)) {
            return -1;
        }
        Node node = cache.get(key);
        moveToHead(node);
        return node.val;
    }

    public void put(int key, int value) {
        if (cache.containsKey(key)) {
            Node node = cache.get(key);
            node.val = value;
            moveToHead(node);
        } else {
            Node node = new Node(key, value);
            cache.put(key, node);
            addToHead(node);
            ++size;
            if (size > capacity) {
                node = removeTail();
                cache.remove(node.key);
                --size;
            }
        }
    }

    private void moveToHead(Node node) {
        removeNode(node);
        addToHead(node);
    }

    private void removeNode(Node node) {
        node.prev.next = node.next;
        node.next.prev = node.prev;
    }

    private void addToHead(Node node) {
        node.next = head.next;
        node.prev = head;
        head.next = node;
        node.next.prev = node;
    }

    private Node removeTail() {
        Node node = tail.prev;
        removeNode(node);
        return node;
    }
}

/**
 * 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);
 */

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struct Node {
    int k;
    int v;
    Node* prev;
    Node* next;

    Node()
        : k(0)
        , v(0)
        , prev(nullptr)
        , next(nullptr) {}
    Node(int key, int val)
        : k(key)
        , v(val)
        , prev(nullptr)
        , next(nullptr) {}
};

class LRUCache {
public:
    LRUCache(int capacity)
        : cap(capacity)
        , size(0) {
        head = new Node();
        tail = new Node();
        head->next = tail;
        tail->prev = head;
    }

    int get(int key) {
        if (!cache.count(key)) return -1;
        Node* node = cache[key];
        moveToHead(node);
        return node->v;
    }

    void put(int key, int value) {
        if (cache.count(key)) {
            Node* node = cache[key];
            node->v = value;
            moveToHead(node);
        } else {
            Node* node = new Node(key, value);
            cache[key] = node;
            addToHead(node);
            ++size;
            if (size > cap) {
                node = removeTail();
                cache.erase(node->k);
                --size;
            }
        }
    }

private:
    unordered_map<int, Node*> cache;
    Node* head;
    Node* tail;
    int cap;
    int size;

    void moveToHead(Node* node) {
        removeNode(node);
        addToHead(node);
    }

    void removeNode(Node* node) {
        node->prev->next = node->next;
        node->next->prev = node->prev;
    }

    void addToHead(Node* node) {
        node->next = head->next;
        node->prev = head;
        head->next = node;
        node->next->prev = node;
    }

    Node* removeTail() {
        Node* node = tail->prev;
        removeNode(node);
        return node;
    }
};

/**
 * 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);
 */

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type node struct {
    key, val   int
    prev, next *node
}

type LRUCache struct {
    capacity   int
    cache      map[int]*node
    head, tail *node
}

func Constructor(capacity int) LRUCache {
    head := new(node)
    tail := new(node)
    head.next = tail
    tail.prev = head
    return LRUCache{
        capacity: capacity,
        cache:    make(map[int]*node, capacity),
        head:     head,
        tail:     tail,
    }
}

func (this *LRUCache) Get(key int) int {
    n, ok := this.cache[key]
    if !ok {
        return -1
    }
    this.moveToFront(n)
    return n.val
}

func (this *LRUCache) Put(key int, value int) {
    n, ok := this.cache[key]
    if ok {
        n.val = value
        this.moveToFront(n)
        return
    }
    if len(this.cache) == this.capacity {
        back := this.tail.prev
        this.remove(back)
        delete(this.cache, back.key)
    }
    n = &node{key: key, val: value}
    this.pushFront(n)
    this.cache[key] = n
}

func (this *LRUCache) moveToFront(n *node) {
    this.remove(n)
    this.pushFront(n)
}

func (this *LRUCache) remove(n *node) {
    n.prev.next = n.next
    n.next.prev = n.prev
    n.prev = nil
    n.next = nil
}

func (this *LRUCache) pushFront(n *node) {
    n.prev = this.head
    n.next = this.head.next
    this.head.next.prev = n
    this.head.next = n
}

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class LRUCache {
    capacity: number;
    map: Map<number, number>;
    constructor(capacity: number) {
        this.capacity = capacity;
        this.map = new Map();
    }

    get(key: number): number {
        if (this.map.has(key)) {
            const val = this.map.get(key)!;
            this.map.delete(key);
            this.map.set(key, val);
            return val;
        }
        return -1;
    }

    put(key: number, value: number): void {
        this.map.delete(key);
        this.map.set(key, value);
        if (this.map.size > this.capacity) {
            this.map.delete(this.map.keys().next().value);
        }
    }
}

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

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use std::cell::RefCell;
use std::collections::HashMap;
use std::rc::Rc;

struct Node {
    key: i32,
    value: i32,
    prev: Option<Rc<RefCell<Node>>>,
    next: Option<Rc<RefCell<Node>>>,
}

impl Node {
    #[inline]
    fn new(key: i32, value: i32) -> Self {
        Self {
            key,
            value,
            prev: None,
            next: None,
        }
    }
}

struct LRUCache {
    capacity: usize,
    cache: HashMap<i32, Rc<RefCell<Node>>>,
    head: Option<Rc<RefCell<Node>>>,
    tail: Option<Rc<RefCell<Node>>>,
}

/**
 * `&self` means the method takes an immutable reference.
 * If you need a mutable reference, change it to `&mut self` instead.
 */
impl LRUCache {
    fn new(capacity: i32) -> Self {
        Self {
            capacity: capacity as usize,
            cache: HashMap::new(),
            head: None,
            tail: None,
        }
    }

    fn get(&mut self, key: i32) -> i32 {
        match self.cache.get(&key) {
            Some(node) => {
                let node = Rc::clone(node);
                self.remove(&node);
                self.push_front(&node);
                let value = node.borrow().value;
                value
            }
            None => -1,
        }
    }

    fn put(&mut self, key: i32, value: i32) {
        match self.cache.get(&key) {
            Some(node) => {
                let node = Rc::clone(node);
                node.borrow_mut().value = value;
                self.remove(&node);
                self.push_front(&node);
            }
            None => {
                let node = Rc::new(RefCell::new(Node::new(key, value)));
                self.cache.insert(key, Rc::clone(&node));
                self.push_front(&node);
                if self.cache.len() > self.capacity {
                    let back_key = self.pop_back().unwrap().borrow().key;
                    self.cache.remove(&back_key);
                }
            }
        };
    }

    fn push_front(&mut self, node: &Rc<RefCell<Node>>) {
        match self.head.take() {
            Some(head) => {
                head.borrow_mut().prev = Some(Rc::clone(node));
                node.borrow_mut().prev = None;
                node.borrow_mut().next = Some(head);
                self.head = Some(Rc::clone(node));
            }
            None => {
                self.head = Some(Rc::clone(node));
                self.tail = Some(Rc::clone(node));
            }
        };
    }

    fn remove(&mut self, node: &Rc<RefCell<Node>>) {
        match (node.borrow().prev.as_ref(), node.borrow().next.as_ref()) {
            (None, None) => {
                self.head = None;
                self.tail = None;
            }
            (None, Some(next)) => {
                self.head = Some(Rc::clone(next));
                next.borrow_mut().prev = None;
            }
            (Some(prev), None) => {
                self.tail = Some(Rc::clone(prev));
                prev.borrow_mut().next = None;
            }
            (Some(prev), Some(next)) => {
                next.borrow_mut().prev = Some(Rc::clone(prev));
                prev.borrow_mut().next = Some(Rc::clone(next));
            }
        };
    }

    fn pop_back(&mut self) -> Option<Rc<RefCell<Node>>> {
        match self.tail.take() {
            Some(tail) => {
                self.remove(&tail);
                Some(tail)
            }
            None => None,
        }
    }
}

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public class LRUCache {
    class Node {
        public Node Prev;
        public Node Next;
        public int Key;
        public int Val;
    }

    private Node head = new Node();
    private Node tail = new Node();
    private Dictionary<int, Node> cache = new Dictionary<int, Node>();
    private readonly int capacity;
    private int size;

    public LRUCache(int capacity) {
        this.capacity = capacity;
        head.Next = tail;
        tail.Prev = head;
    }

    public int Get(int key) {
        Node node;
        if (cache.TryGetValue(key, out node)) {
            moveToHead(node);
            return node.Val;
        }
        return -1;
    }

    public void Put(int key, int Val) {
        Node node;
        if (cache.TryGetValue(key, out node)) {
            moveToHead(node);
            node.Val = Val;
        } else {
            node = new Node() { Key = key, Val = Val };
            cache.Add(key, node);
            addToHead(node);
            if (++size > capacity) {
                node = removeTail();
                cache.Remove(node.Key);
                --size;
            }
        }
    }

    private void moveToHead(Node node) {
        removeNode(node);
        addToHead(node);
    }

    private void removeNode(Node node) {
        node.Prev.Next = node.Next;
        node.Next.Prev = node.Prev;
    }

    private void addToHead(Node node) {
        node.Next = head.Next;
        node.Prev = head;
        head.Next = node;
        node.Next.Prev = node;
    }

    private Node removeTail() {
        Node node = tail.Prev;
        removeNode(node);
        return node;
    }
}

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

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