LeetCode 0460. LFU Cache Solution in Java, C++, Python & More | Explanation + Code

Description

Design and implement a data structure for a Least Frequently Used (LFU) cache.

Implement the LFUCache class:

• LFUCache(int capacity) Initializes the object with the capacity of the data structure.
• int get(int key) Gets the value of the key if the key exists in the cache. Otherwise, returns -1.
• void put(int key, int value) Update the value of the key if present, or inserts the key if not already present. When the cache reaches its capacity, it should invalidate and remove the least frequently used key before inserting a new item. For this problem, when there is a tie (i.e., two or more keys with the same frequency), the least recently used key would be invalidated.

To determine the least frequently used key, a use counter is maintained for each key in the cache. The key with the smallest use counter is the least frequently used key.

When a key is first inserted into the cache, its use counter is set to 1 (due to the put operation). The use counter for a key in the cache is incremented either a get or put operation is called on it.

The functions get and put must each run in O(1) average time complexity.

 

Example 1:

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

Explanation
// cnt(x) = the use counter for key x
// cache=[] will show the last used order for tiebreakers (leftmost element is  most recent)
LFUCache lfu = new LFUCache(2);
lfu.put(1, 1);   // cache=[1,_], cnt(1)=1
lfu.put(2, 2);   // cache=[2,1], cnt(2)=1, cnt(1)=1
lfu.get(1);      // return 1
                 // cache=[1,2], cnt(2)=1, cnt(1)=2
lfu.put(3, 3);   // 2 is the LFU key because cnt(2)=1 is the smallest, invalidate 2.
                 // cache=[3,1], cnt(3)=1, cnt(1)=2
lfu.get(2);      // return -1 (not found)
lfu.get(3);      // return 3
                 // cache=[3,1], cnt(3)=2, cnt(1)=2
lfu.put(4, 4);   // Both 1 and 3 have the same cnt, but 1 is LRU, invalidate 1.
                 // cache=[4,3], cnt(4)=1, cnt(3)=2
lfu.get(1);      // return -1 (not found)
lfu.get(3);      // return 3
                 // cache=[3,4], cnt(4)=1, cnt(3)=3
lfu.get(4);      // return 4
                 // cache=[4,3], cnt(4)=2, cnt(3)=3

 

Constraints:

• 1 <= capacity <= 104
• 0 <= key <= 105
• 0 <= value <= 109
• At most 2 * 105 calls will be made to get and put.

 

 

Solutions

Solution 1

PythonJavaC++GoRust

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 class DoublyLinkedList: def __init__(self) -> None: self.head = Node(-1, -1) self.tail = Node(-1, -1) self.head.next = self.tail self.tail.prev = self.head def add_first(self, node: Node) -> None: node.prev = self.head node.next = self.head.next self.head.next.prev = node self.head.next = node 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: return self.head.next == self.tail class LFUCache: def __init__(self, capacity: int): self.capacity = capacity self.min_freq = 0 self.map = defaultdict(Node) self.freq_map = defaultdict(DoublyLinkedList) 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.add_node(node) 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 self.add_node(node) def add_node(self, node: Node) -> None: freq = node.freq ls = self.freq_map[freq] ls.add_first(node) 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)(code-box)

class LFUCache { private final Map<Integer, Node> map; private final Map<Integer, DoublyLinkedList> freqMap; 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) { DoublyLinkedList list = freqMap.get(minFreq); map.remove(list.removeLast().key); } Node node = new Node(key, value); addNode(node); map.put(key, node); minFreq = 1; } private void incrFreq(Node node) { int freq = node.freq; DoublyLinkedList list = freqMap.get(freq); list.remove(node); if (list.isEmpty()) { freqMap.remove(freq); if (freq == minFreq) { minFreq++; } } node.freq++; addNode(node); } private void addNode(Node node) { int freq = node.freq; DoublyLinkedList list = freqMap.getOrDefault(freq, new DoublyLinkedList()); list.addFirst(node); 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 static class DoublyLinkedList { private final Node head; private final Node tail; public DoublyLinkedList() { head = new Node(-1, -1); tail = new Node(-1, -1); head.next = tail; tail.prev = head; } public void addFirst(Node node) { node.prev = head; node.next = head.next; head.next.prev = node; head.next = node; } 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() { return head.next == tail; } } }(code-box)

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; } }; class DoublyLinkedList { public: Node* head; Node* tail; DoublyLinkedList() { this->head = new Node(-1, -1); this->tail = new Node(-1, -1); this->head->next = this->tail; this->tail->prev = this->head; } void addFirst(Node* node) { node->prev = this->head; node->next = this->head->next; this->head->next->prev = node; this->head->next = node; } 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() { return this->head->next == this->tail; } }; 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) { DoublyLinkedList* list = freqMap[minFreq]; Node* node = list->removeLast(); map.erase(node->key); } Node* node = new Node(key, value); addNode(node); map[key] = node; minFreq = 1; } private: int capacity; int minFreq; unordered_map<int, Node*> map; unordered_map<int, DoublyLinkedList*> freqMap; void incrFreq(Node* node) { int freq = node->freq; DoublyLinkedList* list = freqMap[freq]; list->remove(node); if (list->isEmpty()) { freqMap.erase(freq); if (freq == minFreq) { minFreq++; } } node->freq++; addNode(node); } void addNode(Node* node) { int freq = node->freq; if (freqMap.find(freq) == freqMap.end()) { freqMap[freq] = new DoublyLinkedList(); } DoublyLinkedList* list = freqMap[freq]; list->addFirst(node); 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); */(code-box)

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.addNode(n) 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++ this.addNode(n) } 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 { head *node tail *node } func newList() *list { head := new(node) tail := new(node) head.next = tail tail.prev = head return &list{ head: head, tail: tail, } } func (l *list) pushFront(n *node) { n.prev = l.head n.next = l.head.next l.head.next.prev = n l.head.next = n } 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 { return l.head.next == l.tail }(code-box)

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, } } } struct LinkedList { head: Option<Rc<RefCell<Node>>>, tail: Option<Rc<RefCell<Node>>>, } impl LinkedList { fn new() -> Self { Self { head: None, tail: None, } } 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 => { node.borrow_mut().prev = None; node.borrow_mut().next = 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, } } fn is_empty(&self) -> bool { self.head.is_none() } } struct LFUCache { cache: HashMap<i32, Rc<RefCell<Node>>>, freq_map: HashMap<i32, LinkedList>, 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.add_node(&node); 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; self.add_node(node); } 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 => { let mut list = LinkedList::new(); list.push_front(node); self.freq_map.insert(node.borrow().freq, list); } }; } }(code-box)