[LeetCode] LRU Cache

Problem:

Design and implement a data structure for Least Recently Used (LRU) cache. It should support the following operations: get and set.

get(key) - Get the value (will always be positive) of the key if the key exists in the cache, otherwise return -1.
set(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.

Solution:O(1)

public class LRUCache {  
    private int capacity;  
    private HashMap map = new HashMap<>();  
    private Node head = new Node(-1, -1);  
    private Node tail = new Node(-2, -2);  
  
    public LRUCache(int capacity) {  
        this.capacity = capacity;  
        head.next = tail;  
        tail.pre = head;  
    }  
  
    public int get(int key) {  
        if (!map.containsKey(key))  
            return -1;  
        Node cur = map.get(key);  
        cur.pre.next = cur.next;  
        cur.next.pre = cur.pre;  
        attach(cur);  
        return cur.value;  
    }  
  
    public void set(int key, int value) {  
        if (map.containsKey(key)) {  
            Node cur = map.get(key);  
            cur.value = value;  
            cur.pre.next = cur.next;  
            cur.next.pre = cur.pre;  
            attach(cur);  
        } else {  
            if (map.size() == capacity)  
                dettach();  
            Node cur = new Node(key, value);  
            attach(cur);  
            map.put(key, cur);  
        }  
    }  
      
    public void dettach(){  
        Node last = tail.pre;  
        last.pre.next = last.next;  
        last.next.pre = last.pre;  
        map.remove(last.key);  
    }  
      
    public void attach(Node cur){  
        cur.next = head.next;  
        cur.next.pre = cur;  
        head.next = cur;  
        cur.pre = head;  
    }  
  
    class Node {  
        public Node pre = null;  
        public int key;  
        public int value;  
        public Node next = null;  
  
        Node(int key, int value) {  
            this.key = key;  
            this.value = value;  
        }  
    }  
}