Leetcode: LRU Cache

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.

public class LRUCache {private int capacity;private HashMap<Integer, Node> map = new HashMap<Integer, Node>();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;}private 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;}}}