146. LRU缓存机制

LRU的思路：

​ 查找某个节点，如果该节点存在，则将它放到头部

​ 增加一个节点，如果不存在，将它添加到头部，如果此时超过了LRU的大小，则将最后一个节点删除；如果存在，就更新节点的value，然后放到头部。

实现方式可以用LinkedHashMap，也可以自己实现

LinkedHashMap：

class LRUCache extends LinkedHashMap<Integer,Integer> {

    private int capacity;
    public LRUCache(int capacity) {
        super(capacity,0.75F,true);
        this.capacity=capacity;
    }
    
    public int get(int key) {
        return super.getOrDefault(key,-1);
    }
    
    public void put(int key, int value) {
        super.put(key,value);
    }

    @Override
    protected boolean removeEldestEntry(Map.Entry<Integer,Integer> eldest) {
        return size()>capacity;
    }
}

/**
 * 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);
 */

手动实现 hash + 链表

import java.util.Hashtable;

class LRUCache {

    class DLinkedNode {
        int key;
        int value;
        DLinkedNode prev;
        DLinkedNode next;
    }
    
    private Hashtable<Integer, DLinkedNode> cache = new Hashtable<Integer, DLinkedNode>();
    private int size;
    private int capacity;
    private DLinkedNode head, tail;

    public LRUCache(int capacity) {
        this.size=0;
        this.capacity = capacity;
        head = new DLinkedNode();
        tail = new DLinkedNode();
        head.next = tail;
        tail.prev = head;
    }
    
    public int get(int key) {
        DLinkedNode node = cache.get(key);
        if (node == null) {
            return -1;
        }
        moveToHead(node);
        return node.value;
    }
    
    public void put(int key, int value) {
        DLinkedNode node = cache.get(key);
        if (node == null) {
            DLinkedNode newNode = new DLinkedNode();
            newNode.key = key;
            newNode.value = value;
            
            cache.put(key, newNode);
            addNode(newNode);
            ++size;
            
            if (size>capacity) {
                DLinkedNode _tail = popTail();
                cache.remove(_tail.key);
                --size;
            }
        } else {
            node.value = value;
            moveToHead(node);
        }
    }
    
    private void addNode(DLinkedNode node) {
        node.prev = head;
        node.next = head.next;
        head.next.prev = node;
        head.next = node;
    }
    
    private void removeNode(DLinkedNode node) {
        node.next.prev = node.prev;
        node.prev.next = node.next;
    }
    
    private void moveToHead(DLinkedNode node) {
        removeNode(node);
        addNode(node);
    }
    
    private DLinkedNode popTail() {
        DLinkedNode res = tail.prev;
        removeNode(res);
        return res;
    }
}

/**
 * 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);
 */