README_EN.md

16.25. LRU Cache

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Description

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

Solutions

Python3

class Node:
    def __init__(self, key=0, value=0):
        self.key = key
        self.value = value
        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.value

    def put(self, key: int, value: int) -> None:
        if key in self.cache:
            node = self.cache[key]
            node.value = 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
        self.head.next.prev = node
        self.head.next = node
        node.prev = self.head

    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)

Java

class LRUCache {
    class Node {
        int key;
        int value;
        Node prev;
        Node next;
        Node() {
        }
        Node(int key, int value) {
            this.key = key;
            this.value = value;
        }
    }

    private Map<Integer, Node> cache;
    private Node head;
    private Node tail;
    private int capacity;
    private int size;

    public LRUCache(int capacity) {
        cache = new HashMap<>();
        this.capacity = capacity;
        head = new Node();
        tail = new Node();
        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.value;
    }

    public void put(int key, int value) {
        if (cache.containsKey(key)) {
            Node node = cache.get(key);
            node.value = 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;
        head.next.prev = node;
        head.next = node;
        node.prev = head;
    }

    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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