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Formatted question description: https://leetcode.ca/all/460.html

# 460. LFU Cache

Hard

## Description

Design and implement a data structure for Least Frequently Used (LFU) cache. It should support the following operations: get and put.

• get(key) - Get the value (will always be positive) of the key if the key exists in the cache, otherwise return -1.
• put(key, value) - Set or insert the value if the key is not already present. When the cache reaches its capacity, it should invalidate the least frequently used item before inserting a new item. For the purpose of this problem, when there is a tie (i.e., two or more keys that have the same frequency), the least recently used key would be evicted.

Note that the number of times an item is used is the number of calls to the get and put functions for that item since it was inserted. This number is set to zero when the item is removed.

Could you do both operations in O(1) time complexity?

Example:

LFUCache cache = new LFUCache( 2 /* capacity */ );

cache.put(1, 1);
cache.put(2, 2);
cache.get(1);       // returns 1
cache.put(3, 3);    // evicts key 2
cache.get(3);       // returns 3.
cache.put(4, 4);    // evicts key 1.
cache.get(3);       // returns 3
cache.get(4);       // returns 4


## Solution

Create two classes Node and DoublyLinkedList. Each object of Node has a key, a value, a frequency, the previous node and the next node. Each object of DoublyLinkedList has a size, a head and a tail.

In class LFUCache, maintain two maps cache and frequencyMap, which stores each key and the corresponding node, and stores each frequency and the corresponding doubly linked lists, respectively. Also maintain size, capacity and minCapacity.

For the constructor, initialize the two maps and the capacity.

For method get, get the node from cache. If the node exists, update the node’s frequency and return the node’s value. Otherwise, return -1.

For method put, check whether the node of the key already exists. If the node already exists, update the node’s value and update the node’s frequency. Otherwise, if size already reaches compacity, obtain the minimum frequency list using minFrequency and remove the last node, and decreasesize by 1. Then create a new node using key and value, update the maps, increase size by 1 and set minFrequency = 1.

• 
public class LFU_Cache {

public class LFUCache {

// Save the key, value
HashMap<Integer, Integer> vals;

// Save the key to the value of the number of visits
HashMap<Integer, Integer> counts;

// 频率和一个里面所有key都是当前频率的list之间的映射

int capacity;

// Initialize the frequency of data occurrences
int min = -1;

public LFUCache(int cap) {
capacity = cap;
vals = new HashMap<>();
counts = new HashMap<>();
lists = new HashMap<>();
}

public int get(int key) {
if (!vals.containsKey(key))
return -1;

int count = counts.get(key);
counts.put(key, count + 1);
lists.get(count).remove(key);

// Determine whether min should add 1
if (count == min && lists.get(count).size() == 0) {
min++;
}

if (!lists.containsKey(count + 1)) {
}

return vals.get(key);
}

public void put(int key, int value) {
if (capacity <= 0)
return;

if (vals.containsKey(key)) {
vals.put(key, value);
get(key);
return;
}

if (vals.size() >= capacity) {
int minFreKey = lists.get(min).iterator().next();
lists.get(min).remove(minFreKey);
vals.remove(minFreKey);
counts.remove(minFreKey);
}

vals.put(key, value);
counts.put(key, 1);
min = 1;
if (!lists.containsKey(1)) {
}
}
}
}

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

class LFUCache {

private final Map<Integer, Node> map;
private final int capacity;
private int minFreq;

public LFUCache(int capacity) {
this.capacity = capacity;
map = new HashMap<>(capacity, 1);
freqMap = new HashMap<>();
}

public int get(int key) {
if (capacity == 0) {
return -1;
}
if (!map.containsKey(key)) {
return -1;
}
Node node = map.get(key);
incrFreq(node);
return node.value;
}

public void put(int key, int value) {
if (capacity == 0) {
return;
}
if (map.containsKey(key)) {
Node node = map.get(key);
node.value = value;
incrFreq(node);
return;
}
if (map.size() == capacity) {
map.remove(list.removeLast().key);
}
Node node = new Node(key, value);
map.put(key, node);
minFreq = 1;
}

private void incrFreq(Node node) {
int freq = node.freq;
list.remove(node);
if (list.isEmpty()) {
freqMap.remove(freq);
if (freq == minFreq) {
minFreq++;
}
}
node.freq++;
}

int freq = node.freq;
freqMap.put(freq, list);
}

private static class Node {
int key;
int value;
int freq;
Node prev;
Node next;

Node(int key, int value) {
this.key = key;
this.value = value;
this.freq = 1;
}
}

private final Node tail;

tail = new Node(-1, -1);
}

}

public Node remove(Node node) {
node.next.prev = node.prev;
node.prev.next = node.next;
node.next = null;
node.prev = null;
return node;
}

public Node removeLast() {
return remove(tail.prev);
}

public boolean isEmpty() {
}
}
}


• from collections import defaultdict

class LFUCache:
def __init__(self, capacity: int):
self.capacity = capacity
self.key_to_value = {}
self.key_to_freq = defaultdict(int)
self.freq_to_keys = defaultdict(OrderedDict)
self.min_freq = 0

def get(self, key: int) -> int:
if key not in self.key_to_value:
return -1

# Update the frequency
self._update_frequency(key)
return self.key_to_value[key]

def put(self, key: int, value: int) -> None:
if self.capacity == 0:
return

if key in self.key_to_value:
# Update the value and frequency
self.key_to_value[key] = value
self._update_frequency(key)
else:
if len(self.key_to_value) >= self.capacity:
# Evict the least frequently used key
self._evict()

# Add the new key-value pair
self.key_to_value[key] = value
self.key_to_freq[key] = 1
self.freq_to_keys[1][key] = None
self.min_freq = 1

def _update_frequency(self, key: int) -> None:
freq = self.key_to_freq[key]
del self.freq_to_keys[freq][key]

if not self.freq_to_keys[freq]:
# If there are no keys with the previous frequency, update min_freq
if self.min_freq == freq:
self.min_freq += 1
del self.freq_to_keys[freq]

freq += 1
self.key_to_freq[key] = freq
self.freq_to_keys[freq][key] = None

def _evict(self) -> None:
keys = self.freq_to_keys[self.min_freq]
evict_key, _ = keys.popitem(last=False)
del self.key_to_value[evict_key]
del self.key_to_freq[evict_key]

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

##########

class Node:
def __init__(self, key: int, value: int) -> None:
self.key = key
self.value = value
self.freq = 1
self.prev = None
self.next = None

def __init__(self) -> None:
self.tail = Node(-1, -1)

def add_first(self, node: Node) -> None:

def remove(self, node: Node) -> Node:
node.next.prev = node.prev
node.prev.next = node.next
node.next, node.prev = None, None
return node

def remove_last(self) -> Node:
return self.remove(self.tail.prev)

def is_empty(self) -> bool:

class LFUCache:
def __init__(self, capacity: int):
self.capacity = capacity
self.min_freq = 0
self.map = defaultdict(Node)

def get(self, key: int) -> int:
if self.capacity == 0 or key not in self.map:
return -1
node = self.map[key]
self.incr_freq(node)
return node.value

def put(self, key: int, value: int) -> None:
if self.capacity == 0:
return
if key in self.map:
node = self.map[key]
node.value = value
self.incr_freq(node)
return
if len(self.map) == self.capacity:
ls = self.freq_map[self.min_freq]
node = ls.remove_last()
self.map.pop(node.key)
node = Node(key, value)
self.map[key] = node
self.min_freq = 1

def incr_freq(self, node: Node) -> None:
freq = node.freq
ls = self.freq_map[freq]
ls.remove(node)
if ls.is_empty():
self.freq_map.pop(freq)
if freq == self.min_freq:
self.min_freq += 1
node.freq += 1

def add_node(self, node: Node) -> None:
freq = node.freq
ls = self.freq_map[freq]
self.freq_map[freq] = ls

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


• type LFUCache struct {
cache    map[int]*node
freqMap  map[int]*list
minFreq  int
capacity int
}

func Constructor(capacity int) LFUCache {
return LFUCache{
cache:    make(map[int]*node),
freqMap:  make(map[int]*list),
capacity: capacity,
}
}

func (this *LFUCache) Get(key int) int {
if this.capacity == 0 {
return -1
}

n, ok := this.cache[key]
if !ok {
return -1
}

this.incrFreq(n)
return n.val
}

func (this *LFUCache) Put(key int, value int) {
if this.capacity == 0 {
return
}

n, ok := this.cache[key]
if ok {
n.val = value
this.incrFreq(n)
return
}

if len(this.cache) == this.capacity {
l := this.freqMap[this.minFreq]
delete(this.cache, l.removeBack().key)
}
n = &node{key: key, val: value, freq: 1}
this.cache[key] = n
this.minFreq = 1
}

func (this *LFUCache) incrFreq(n *node) {
l := this.freqMap[n.freq]
l.remove(n)
if l.empty() {
delete(this.freqMap, n.freq)
if n.freq == this.minFreq {
this.minFreq++
}
}
n.freq++
}

func (this *LFUCache) addNode(n *node) {
l, ok := this.freqMap[n.freq]
if !ok {
l = newList()
this.freqMap[n.freq] = l
}
l.pushFront(n)
}

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

type list struct {
tail *node
}

func newList() *list {
tail := new(node)
return &list{
tail: tail,
}
}

func (l *list) pushFront(n *node) {
}

func (l *list) remove(n *node) {
n.prev.next = n.next
n.next.prev = n.prev
n.next = nil
n.prev = nil
}

func (l *list) removeBack() *node {
n := l.tail.prev
l.remove(n)
return n
}

func (l *list) empty() bool {
}


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

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

impl Node {
fn new(key: i32, value: i32) -> Self {
Self {
key,
value,
freq: 1,
prev: None,
next: None,
}
}
}

tail: Option<Rc<RefCell<Node>>>,
}

fn new() -> Self {
Self {
tail: None,
}
}

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

fn is_empty(&self) -> bool {
}
}

struct LFUCache {
cache: HashMap<i32, Rc<RefCell<Node>>>,
min_freq: i32,
capacity: usize,
}

/**
* &self means the method takes an immutable reference.
* If you need a mutable reference, change it to &mut self instead.
*/
impl LFUCache {
fn new(capacity: i32) -> Self {
Self {
cache: HashMap::new(),
freq_map: HashMap::new(),
min_freq: 0,
capacity: capacity as usize,
}
}

fn get(&mut self, key: i32) -> i32 {
if self.capacity == 0 {
return -1;
}

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

fn put(&mut self, key: i32, value: i32) {
if self.capacity == 0 {
return;
}

match self.cache.get(&key) {
Some(node) => {
let node = Rc::clone(node);
node.borrow_mut().value = value;
self.incr_freq(&node);
}
None => {
if self.cache.len() == self.capacity {
let list = self.freq_map.get_mut(&self.min_freq).unwrap();
self.cache.remove(&list.pop_back().unwrap().borrow().key);
}
let node = Rc::new(RefCell::new(Node::new(key, value)));
self.cache.insert(key, node);
self.min_freq = 1;
}
};
}

fn incr_freq(&mut self, node: &Rc<RefCell<Node>>) {
let freq = node.borrow().freq;
let list = self.freq_map.get_mut(&freq).unwrap();
list.remove(node);
if list.is_empty() {
self.freq_map.remove(&freq);
if freq == self.min_freq {
self.min_freq += 1;
}
}
node.borrow_mut().freq += 1;
}

fn add_node(&mut self, node: &Rc<RefCell<Node>>) {
let freq = node.borrow().freq;
match self.freq_map.get_mut(&freq) {
Some(list) => {
list.push_front(node);
}
None => {
list.push_front(node);
self.freq_map.insert(node.borrow().freq, list);
}
};
}
}

/**
* Your LFUCache object will be instantiated and called as such:
* let obj = LFUCache::new(capacity);
* let ret_1: i32 = obj.get(key);
* obj.put(key, value);
*/


• class Node {
public:
int key;
int value;
int freq;
Node* prev;
Node* next;
Node(int key, int value) {
this->key = key;
this->value = value;
this->freq = 1;
this->prev = nullptr;
this->next = nullptr;
}
};

public:
Node* tail;
this->tail = new Node(-1, -1);
}
}
Node* remove(Node* node) {
node->next->prev = node->prev;
node->prev->next = node->next;
node->next = nullptr;
node->prev = nullptr;
return node;
}
Node* removeLast() {
return remove(this->tail->prev);
}
bool isEmpty() {
}
};

class LFUCache {
public:
LFUCache(int capacity) {
this->capacity = capacity;
this->minFreq = 0;
}

int get(int key) {
if (capacity == 0 || map.find(key) == map.end()) {
return -1;
}
Node* node = map[key];
incrFreq(node);
return node->value;
}

void put(int key, int value) {
if (capacity == 0) {
return;
}
if (map.find(key) != map.end()) {
Node* node = map[key];
node->value = value;
incrFreq(node);
return;
}
if (map.size() == capacity) {
Node* node = list->removeLast();
map.erase(node->key);
}
Node* node = new Node(key, value);
map[key] = node;
minFreq = 1;
}

private:
int capacity;
int minFreq;
unordered_map<int, Node*> map;

void incrFreq(Node* node) {
int freq = node->freq;
list->remove(node);
if (list->isEmpty()) {
freqMap.erase(freq);
if (freq == minFreq) {
minFreq++;
}
}
node->freq++;
}

int freq = node->freq;
if (freqMap.find(freq) == freqMap.end()) {
}
freqMap[freq] = list;
}
};

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