（1） super(chcheSize, 0.75f, true)， 第三个参数必须是true，表示按照访问顺序控制LinkedHashMap元素顺序，默认是false按照插入顺序排序。ture情况下访问元素后该元素会被放到末尾，因此头部都是最近最久没有访问的元素；
（2）removeEldestEntry是否删除eldestEntry
用双向链表加Map可以自己实现lru
class LRUCache extends LinkedHashMap<Integer, Integer> { private int capacity; public LRUCache(int capacity){ super(capacity,0.75f,true); this.capacity = capacity; } @Override protected boolean removeEldestEntry(Map.Entry<Integer, Integer> oldestEntry){ return size() > capacity; } //put可以不写，但是get要写，否则不存在的时候返回的是null而不是-1，不符合题目要求，实际上是可以不写的 public int get(int key) { return super.getOrDefault(key, -1); } /* private int capacity; private DoubleLinkedList linkedList; private Map<Integer, Node> map; public LRUCache(int capacity) { if (capacity <= 0) { throw new IllegalArgumentException("Invalid capacity"); } this.capacity = capacity; this.linkedList = new DoubleLinkedList(); this.map = new HashMap<>(); } public int get(int key) { if (!map.containsKey(key)) { return -1; } Node node = map.get(key); linkedList.remove(node); linkedList.add(node); return node.value; } public void put(int key, int value) { if (map.containsKey(key)) { linkedList.remove(map.get(key)); } if (this.capacity == linkedList.size()) { Node oldestNode = linkedList.peek(); linkedList.remove(oldestNode); map.remove(oldestNode.key); } Node node = new Node(key, value); linkedList.add(node); map.put(key, node); } private class Node { private int key; private int value; private Node prev = null; private Node next = null; Node(int key, int value) { this.key = key; this.value = value; } } private class DoubleLinkedList { private int size; private Node dummyHead; private Node dummyTail; DoubleLinkedList() { this.size = 0; this.dummyHead = new Node(0, 0); this.dummyTail = new Node(0, 0); dummyHead.next = dummyTail; dummyTail.prev = dummyHead; } int size() { return this.size; } Node peek() { return this.size == 0 ? null : dummyHead.next; } void add(Node node) { node.next = dummyTail; node.prev = dummyTail.prev; dummyTail.prev.next = node; dummyTail.prev = node; size++; } void remove(Node node) { node.prev.next = node.next; node.next.prev = node.prev; size--; } } */ }
