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.put(4, 4); // LRU key was 1, evicts key 1, cache is {4=4, 3=3}
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

Create a class MyNode, which contains data fields int key, int value, Node prev and Node next. That is, each node of type MyNode has a key and a value, and have references to its previous node and the next node.

In class LRUCache, data fields include int capacity that stores the capacity of the cache, Map<Integer, MyNode> map that maps each key to its node, MyNode head and Node tail that represents the head node and the tail node respectively. For Least Recently Used cache, the most recently used node is the head node and the least recently used node is the tail node.

In the constructor, initialize capacity with the given capacity.

In get(key), if key is not in map, then key is not in the cache, so return -1. If key is in map, obtain the node and its value, remove the node and set the node to be the head, and return value.

In put(key, value), if map contains key, then obtain the node and update its value, remove the node, and set the node to be the head. If map does not contain key, then create a new node using key and value, and set the new node to be the head. If the size of map is greater than or equal to capacity, then remove the node tail and remove the corresponding entry in map. Add a new entry of the new node into the map.

Two supplementary methods are needed.

1. Method remove(MyNode node). Obtain MyNode’s previous node and next node, and update their references to other nodes accordingly. If MyNode is head or tail, then update head or tail accordingly.
2. Method setHead(MyNode node). Set MyNode to be the new head and set the previous head’s reference accordingly. If tail is null, then update tail as well.
• 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;
}

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

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

removeNode(node);
}

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

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) {
tail = new Node();
}

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

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

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

removeNode(node);
}

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

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);
*/

• 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 = {} # key ==> Node(val)
self.head = Node() # dummy node
self.tail = Node() # dummy node
self.capacity = capacity
self.size = 0
self.head.next = self.tail # note: key setup

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

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

self.remove_node(node)

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

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)

############

'''
example:

>>> od = collections.OrderedDict()
>>>
>>> od[1]=1
>>> od[2]=2
>>> od[3]=3
>>>
>>> od
OrderedDict([(1, 1), (2, 2), (3, 3)])
>>> od.move_to_end(1)
>>> od
OrderedDict([(2, 2), (3, 3), (1, 1)])
>>>
>>> od.get(1)
1
>>> od.popitem()
(1, 1)
>>> od
OrderedDict([(2, 2), (3, 3)])
>>>
>>> od[1]=1
>>> od
OrderedDict([(2, 2), (3, 3), (1, 1)])
>>>
>>> od.popitem(last=False)
(2, 2)
>>> od
OrderedDict([(3, 3), (1, 1)])

'''

import collections

class LRUCache:
def __init__(self, capacity: 'int'):
self.cache = collections.OrderedDict()
self.remain = capacity

def get(self, key: 'int') -> 'int':
if key not in self.cache:
return -1
self.cache.move_to_end(key) # meaning end is the most recently used
return self.cache.get(key)

def put(self, key: 'int', value: 'int') -> 'None':
if key not in self.cache:
if self.remain > 0:
self.remain -= 1
else:
self.cache.popitem(last=False) # pop start position
else:
self.cache.pop(key)
self.cache[key] = value # add to end of dict, meaning most recently used

############

### below solution with no ordered-dict

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

def __init__(self):
self.tail = DLinkedNode() # dummy tail, its prev is real tail

node.next.prev, node.prev.next = node, node

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

def remove_tail(self):
return self.remove_node(self.tail.prev) # dummy tail's prev is real tail

self.remove_node(node)

# def __repr__(self):
#     ans = []
#     while h:
#         ans.append(str(h.val))
#         h = h.next

class LRUCache:

def __init__(self, capacity: int):
self.capacity = capacity
self.nodes_map = {}

def get(self, key: int) -> int:
node = self.nodes_map.get(key, None)
if node:
return node.val
else:
return -1

def put(self, key: int, value: int) -> None:
node = self.nodes_map.get(key, None)
if not node:
if self.capacity > 0:
self.capacity -= 1
else:
rm_key = self.cache_list.remove_tail()
self.nodes_map.pop(rm_key) # note api
self.nodes_map[key] = new_node
else:
node.val = value


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

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

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

• 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)
*/


• 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 int size;

public LRUCache(int capacity) {
this.capacity = capacity;
}

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

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

removeNode(node);
}

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

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);
*/

• 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>>>,
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(),
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>>) {
node.borrow_mut().prev = None;
}
None => {
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.tail = None;
}
(None, Some(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,
}
}
}/**
* Your LRUCache object will be instantiated and called as such:
* let obj = LRUCache::new(capacity);
* let ret_1: i32 = obj.get(key);
* obj.put(key, value);
*/