LRU的几种写法

简介

LRU (Least Recently Used,最近最少使用)算法是操作系统中一种经典的页面置换算法,这么卷的环境下肯定要会手写拉,这篇就是讲茴字的几种写法。

双向链表

简单来说就一个链表,左端入,清理最右,插入与查询都是O(n)

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public class LinkListLRUCache {

/**
* 双端链表
*/
private LinkedList<Node> cache;

/**
* cache大小
*/
private int capacity;

public LRUCache(int capacity) {
this.cache = new LinkedList<>();
this.capacity = capacity;
}

/**
* 获取命中index
*
* @return -1 为找不到
*/
public int get(int key) {
Iterator<Node> iterator = cache.descendingIterator();
int result = -1;
while (iterator.hasNext()) {
Node node = iterator.next();
if (node.key == key) {
result = node.val;
iterator.remove(); // remove node
put(key, result); // reput node
break;
}
}
return result;
}

public void put(int key, int value) {
// 尝试命中已存在并删除对应node
Iterator<Node> iterator = cache.iterator();
while (iterator.hasNext()) {
Node node = iterator.next();
if (node.key == key) {
iterator.remove();
break;
}
}

if (capacity == cache.size()) {
// 缓存满,删除链表头
cache.removeFirst();
}
cache.add(new Node(key, value));
}


class Node {
int key;
int val;

public Node(int key, int val) {
this.key = key;
this.val = val;
}
}
}

哈希链表

用LinkedHashMap代替LinkedList,插入与查询都是O(1)

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public class HashLinkListLRUCache {

/**
* 哈希链表
*/
private LinkedHashMap<Integer, Integer> cache;


/**
* 缓存容量
*/
private int capacity;

public HashLinkListLRUCache(int capacity) {
this.cache = new LinkedHashMap<>();
this.capacity = capacity;
}

/**
* 获取命中index
*
* @return -1 为找不到
*/
public int get(int key) {
if (!cache.containsKey(key)) return -1;

int val = cache.get(key);
cache.remove(key);
cache.put(key, val);
return val;
}

public void put(int key, int value) {
if (cache.containsKey(key)) {
cache.remove(key);
}

if (capacity == cache.size()) {
Set<Integer> keySet = cache.keySet();
Iterator<Integer> iterator = keySet.iterator();
cache.remove(iterator.next());
}

cache.put(key, value);
}

}
```

当然,也可以重写LinkedHashMap#removeEldestEntry(Map.Entry)
```java
import java.util.LinkedHashMap;
import java.util.Map;

public class HashLinkListLRUCache {

/**
* 哈希链表
*/
private Map<Integer, Integer> cache;

public HashLinkListLRUCache(int capacity) {
this.cache = new LinkedHashMap<>() {
private static final long serialVersionUID = 1L;

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

/**
* 获取命中index
*
* @return -1 为找不到
*/
public int get(int key) {
if (!cache.containsKey(key)) return -1;

int val = cache.get(key);
cache.remove(key);
cache.put(key, val);
return val;
}

public void put(int key, int value) {
if (cache.containsKey(key)) {
cache.remove(key);
}

cache.put(key, value);
}

}

通用一点

改的给阳间一点:

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public class LRUCache<K, V> extends LinkedHashMap<K, V> {

private static final long serialVersionUID = 1L;

/**
* cache容器大小
*/
private int capacity;

/**
* 初始化cache大小
*
* @param capacity cache大小
*/
public LRUCache(int capacity) {
super(capacity);
this.capacity = capacity;
}

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

}