LRU Cache数据结构

leecode上的146题
LRU数据结构是非常有用的数据结构，本题支持有限的队列，并且get和set方法的复杂度为O(1)
Design and implement a data structure for Least Recently Used (LRU) cache. It should support the following operations: get and set.

1. get(key) - Get the value (will always be positive) of the key if the key exists in the cache, otherwise return -1.
2. 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.
   核心要求O(1)，
   核心思想：双向链表+Hashmap，

import java.util.HashMap;public class LRUCache {   int capacity;    HashMap<Integer, Node> map = new HashMap<Integer, Node>();    Node head=null;    Node end=null;    public LRUCache(int capacity) {        this.capacity = capacity;    }    public int get(int key) {        if(map.containsKey(key)){            Node n = map.get(key);            remove(n);            setHead(n);            return n.value;        }        return -1;    }    public void remove(Node n){        if(n.pre!=null){            n.pre.next = n.next;        }else{            head = n.next;        }        if(n.next!=null){            n.next.pre = n.pre;        }else{            end = n.pre;        }    }    public void setHead(Node n){        n.next = head;        n.pre = null;        if(head!=null)            head.pre = n;        head = n;        if(end ==null)            end = head;    }    public void set(int key, int value) {        if(map.containsKey(key)){            Node old = map.get(key);            old.value = value;            remove(old);            setHead(old);        }else{            Node created = new Node(key, value);            if(map.size()>=capacity){                map.remove(end.key);                remove(end);                setHead(created);            }else{                setHead(created);            }                map.put(key, created);        }    }    class Node{        int key;        int value;        Node pre;        Node next;        public Node(int key, int value){            this.key = key;            this.value = value;        }    }}