146. LRU Cache

Design and implement a data structure for Least Recently Used (LRU) 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 reached its capacity, it should invalidate the least recently used item before inserting a new item.

The cache is initialized with a positive capacity.

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

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

Solution

HashMap + 双端链表

Complexity

put O(1)
get O(1)

Code

class LRUCache {
    
    class Node {
        int val;
        int key;
        Node pre;
        Node next;

        public Node(int key, int val, Node pre, Node next) {
            this.key = key;
            this.val = val;
            this.pre = pre;
            this.next = next;
        }
    }

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

    public LRUCache(int capacity) {
        this.capacity = capacity;
        this.cache = new HashMap<>(capacity);
        this.head = new Node(-1, -1, null, null);
        this.tail = new Node(-1, -1, null, null);
        head.next = tail;
        tail.pre = head;
    }

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

    public void put(int key, int value) {
        if (!cache.containsKey(key)) {
            if (cache.size() >= this.capacity) {
                //todo: delete from list
                Node node = tail.pre;
                node.next.pre = node.pre;
                node.pre.next = node.next;
                node.next = null;
                node.pre = null;
                //todo: delete from cache
                cache.remove(node.key);
            }
            Node node = new Node(key, value, null, null);
            cache.put(key, node);
            addFirst(node);
        } else {
            Node node = cache.get(key);
            node.val = value;
            moveToFirst(node);
        }
    }

    public void moveToFirst(Node node) {
        // unbound
        node.pre.next = node.next;
        node.next.pre = node.pre;
        node.pre = null;
        node.next = null;

        addFirst(node);

    }

    public void addFirst(Node node) {
        // move to first
        node.next = head.next;
        head.next.pre = node;

        head.next = node;
        node.pre = head;
    }
    
}

/**
 * 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);
 */