思路：使用hashmap储存key，vaule，使用双向链表以快速查到尾结点（待逐出的节点），链表的题一定要在纸上画一下，不然连着连着就不知道连在哪里去了

class LRUCache {public class ListNode {int key;int value;ListNode next;ListNode pre;ListNode() {}ListNode(int key , int value) { this.key = key; this.value = value;}ListNode(int kye, int value, ListNode next) { this.key = key;this.value = value;this.next = next;}ListNode(int kye, int value, ListNode next,ListNode pre) { this.key = key;this.value = value;this.next = next;this.pre = pre;}}ListNode head;//头节点ListNode tail;//指向尾结点的前一个节点，方便逐出最久未使用关键字int capacity; //储存容量int cur_capacity;//储存当前容量Map<Integer,ListNode> map;//储存节点public LRUCache(int capacity) {this.head = new ListNode(0,0,null,null);this.capacity = capacity;this.cur_capacity = 0;this.map = new HashMap<>();this.tail = new ListNode(0,0,null,head);this.head.next = tail;}public int get(int key) {if(map.containsKey(key))  //最近使用到的key，将位置提前{if(map.get(key).next != null){map.get(key).next.pre = map.get(key).pre;map.get(key).pre.next = map.get(key).next;}map.get(key).next = head.next;map.get(key).next.pre = map.get(key);map.get(key).pre = head;head.next = map.get(key);return map.get(key).value;}elsereturn -1;}public void put(int key, int value) {if(map.containsKey(key)) //存在,将位置提前{map.get(key).value = value;if(map.get(key).next != null){map.get(key).next.pre = map.get(key).pre;map.get(key).pre.next = map.get(key).next;}map.get(key).next = head.next;map.get(key).next.pre = map.get(key);map.get(key).pre = head;head.next = map.get(key);}else{//不存在if(this.cur_capacity < this.capacity) //容量足够，直接添加到头{ListNode temp = new ListNode();temp.key = key;temp.value = value;temp.next = head.next;if(temp.next != null)temp.next.pre = temp;temp.pre = head;head.next = temp;cur_capacity++;map.put(key,temp);}else//容量不够，移出尾结点，添加新节点到头{ListNode tail_pre = tail.pre;map.remove(tail_pre.key);tail.pre.pre.next = tail;tail.pre.next = null;tail.pre = tail.pre.pre;tail_pre.pre = null;ListNode temp = new ListNode();temp.key = key;temp.value = value;temp.next = head.next;temp.next.pre = temp;temp.pre = head;head.next = temp;map.put(key,temp);}}}
}/*** 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);*/