| comments | difficulty | edit_url |
|---|---|---|
true |
Medium |
Design and build a "least recently used" cache, which evicts the least recently used item. The cache should map from keys to values (allowing you to insert and retrieve a value associated with a particular key) and be initialized with a max size. When it is full, it should evict the least recently used item.
You should implement following operations: get and put.
Get a value by key: get(key) - If key is in the cache, return the value, otherwise return -1.
Write a key-value pair to the cache: put(key, value) - If the key is not in the cache, then write its value to the cache. Evict the least recently used item before writing if necessary.
Example:
LRUCache cache = new LRUCache( 2 /* capacity */ ); cache.put(1, 1); cache.put(2, 2); cache.get(1); // returns 1 cache.put(3, 3); // evicts key 2 cache.get(2); // returns -1 (not found) cache.put(4, 4); // evicts key 1 cache.get(1); // returns -1 (not found) cache.get(3); // returns 3 cache.get(4); // returns 4
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
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)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);
*/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);
*/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
}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)
*/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,
}
}
}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);
*/class Node {
var key: Int
var val: Int
var prev: Node?
var next: Node?
init(_ key: Int = 0, _ val: Int = 0) {
self.key = key
self.val = val
}
}
class LRUCache {
private var cache: [Int: Node] = [:]
private let head: Node = Node()
private let tail: Node = Node()
private var capacity: Int
private var size: Int = 0
init(_ capacity: Int) {
self.capacity = capacity
head.next = tail
tail.prev = head
}
func get(_ key: Int) -> Int {
guard let node = cache[key] else {
return -1
}
moveToHead(node)
return node.val
}
func put(_ key: Int, _ value: Int) {
if let node = cache[key] {
node.val = value
moveToHead(node)
} else {
let newNode = Node(key, value)
cache[key] = newNode
addToHead(newNode)
size += 1
if size > capacity {
if let tailNode = removeTail() {
cache.removeValue(forKey: tailNode.key)
size -= 1
}
}
}
}
private func moveToHead(_ node: Node) {
removeNode(node)
addToHead(node)
}
private func removeNode(_ node: Node) {
node.prev?.next = node.next
node.next?.prev = node.prev
}
private func addToHead(_ node: Node) {
node.prev = head
node.next = head.next
head.next?.prev = node
head.next = node
}
private func removeTail() -> Node? {
guard let res = tail.prev, res !== head else {
return nil
}
removeNode(res)
return res
}
}