[算法]链表+HashMap实现LRU算法

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/**
 * @author :dongshuo
 * @date : 2018/12/10 14:27
 * @desc : 链表+hashmap实现的
 */
public class LRUCache1<K,V> {
    private final int MAX_CACHE_SIZE;//最大的缓存大小
    private Entry first; //头元素
    private Entry last;//尾元素

    private HashMap<K, Entry<K, V>> hashMap;//用来保存的hashmap

    /**
     * 初始化 缓存大小和对象
     * @param cacheSize
     */
    public LRUCache1(int cacheSize) {
        MAX_CACHE_SIZE = cacheSize;
        hashMap = new HashMap<K, Entry<K, V>>();
    }

    /**
     * 存入k,v
     * @param key
     * @param value
     */
    public void put(K key, V value) {
        //1首先判断是否存在该值
        Entry entry = getEntry(key);
        //如果为空
        if (entry == null) {
            if (hashMap.size() >= MAX_CACHE_SIZE) {
                hashMap.remove(last.key);
                removeLast();
            }
            entry = new Entry();
            entry.key = key;
        }
        entry.value = value;
        moveToFirst(entry);
        hashMap.put(key, entry);
    }

    public V get(K key) {
        Entry<K, V> entry = getEntry(key);
        if (entry == null) return null;
        moveToFirst(entry);
        return entry.value;
    }

    public void remove(K key) {
        Entry entry = getEntry(key);
        if (entry != null) {
            if (entry.pre != null) entry.pre.next = entry.next;
            if (entry.next != null) entry.next.pre = entry.pre;
            if (entry == first) first = entry.next;
            if (entry == last) last = entry.pre;
        }
        hashMap.remove(key);
    }

    /**
     * 将元素移到first
     * @param entry
     */
    private void moveToFirst(Entry entry) {
        if (entry == first) return;//元素已经在first位置了
        if (entry.pre != null) entry.pre.next = entry.next;
        if (entry.next != null) entry.next.pre = entry.pre;
        if (entry == last) last = last.pre;//元素在last位置

        if (first == null || last == null) {
            first = last = entry;
            return;
        }

        entry.next = first;//元素放到first
        first.pre = entry;
        first = entry;
        entry.pre = null;
    }

    /**
     * 删除最后一个元素
     */
    private void removeLast() {
        if (last != null) {
            last = last.pre;//last元素被删除,last前驱作为最后一个元素
            if (last == null) first = null;//如果 first=last情况时
            else last.next = null;//last后继为null
        }
    }

    /**
     * 判断map是否存在该key,value
     * @param key
     * @return
     */
    private Entry<K, V> getEntry(K key) {
        return hashMap.get(key);
    }

    @Override
    public String toString() {
        StringBuilder sb = new StringBuilder();
        Entry entry = first;
        while (entry != null) {
            sb.append(String.format("%s:%s ", entry.key, entry.value));
            entry = entry.next;
        }
        return sb.toString();
    }

    /**
     * 内部类 链表
     * @param <K>
     * @param <V>
     */
    class Entry<K, V> {
        public Entry pre;
        public Entry next;
        public K key;
        public V value;
    }
}