【算法五十七】146. LRU 缓存

146. LRU 缓存 - 力扣（LeetCode）

HashMap + 双向链表：

class LRUCache {

    class Node {
        int key;
        int value;
        Node prev;
        Node next;

        public Node(){}

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

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

    public LRUCache(int capacity){
        head = new Node();
        tail = new Node();

        this.capacity = capacity;
        this.map = new HashMap<>();

        head.next = tail;
        tail.prev = head;
    }

    public int get(int key){
        Node node = map.get(key);
        if(node == null){
            return -1;
        }
        
        removeToHead(node);
        return node.value;
    }

    public void put(int key,int value){
        Node node = map.get(key);
        if(node != null){
            removeToHead(node);
            node.value = value;
        }
        else{
            Node new_node = new Node();
            new_node.key = key;
            new_node.value = value;
            map.put(key,new_node); 

            moveToHead(new_node);
            if(map.size() > capacity){
                Node last = removeLast();
                map.remove(last.key);
            }
        }
    }

    private Node removeLast(){
        Node node = tail.prev;
        node.prev.next = tail;
        tail.prev = node.prev;
        return node;
    }

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

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