LRU算法（c++）

least recently used（最近最少使用算法），是一种内存数据淘汰策略，使用常见是当内存不足时，需要淘汰最近最少使用的数据。

概括来讲，即，“不断访问，不断缓存，放不下了，得删一个。”

实现方法：hash+双向链表

图片非原创：五分钟学算法 公众号

hash以键找指针，链表节点存键与值

1）读一条数据，hash知道已经在不在，在，挪到链表最后（双向链表才能O(1)），不在，插到链表最后。

2）满了，试图删一条数据，删头元素，更新hash（链表里存键、值二者）

typedef int KEYTYPE;
typedef int VALUETYPE;

class CDoubleListNode {
public:
	KEYTYPE key;
	VALUETYPE value;
	CDoubleListNode * pre;
	CDoubleListNode * nxt;
};

class CLRUpool {
	int capacity;
	int current;
	std::map hashtable;
	CDoubleListNode * pheadNode;
	CDoubleListNode * ptailNode;

public:

	CLRUpool(int _c):capacity(_c), current(0)
	{
		pheadNode = new CDoubleListNode;
		ptailNode = new CDoubleListNode;
		pheadNode->pre = ptailNode->nxt = NULL;
		pheadNode->nxt = ptailNode;
		ptailNode->pre = pheadNode;
	}

	void find(KEYTYPE key)
	{
		std::map::iterator it; //第二个双冒号啥意思
		it = hashtable.find(key);

		if (it == hashtable.end()) 
		{
			std::cout << "**未查到信息" << key << "，请输入其value:**" << std::endl;
			int value;
			std::cin >> value;
			add(key, value);
		}
		else
		{
			std::cout << "**查到" << key << "的value是" << (it->second)->value << "**" << std::endl;
			// *it指向的节点移到最后
			CDoubleListNode * tmp = it->second;
			tmp->pre->nxt = tmp->nxt;
			tmp->nxt = tmp->pre;
			tmp->pre = ptailNode->pre;
			tmp->nxt = ptailNode;
			ptailNode->pre->nxt = tmp;
			ptailNode->pre = tmp;
		}
		return;
	}

	void add(KEYTYPE key, VALUETYPE value)
	{
		if (current == capacity)
		{
			// 删头
			std::cout << "**缓存已满，删除" << pheadNode->nxt->key << std::endl;

			CDoubleListNode * tmp = pheadNode->nxt;
			hashtable.erase(tmp->key);
			pheadNode->nxt = tmp->nxt;
			tmp->nxt->pre = pheadNode;
			delete tmp;
		}

		// 添加
		std::cout << "**已添加" << key << '-' << value << "**" << std::endl;
		
		CDoubleListNode * tmp = new CDoubleListNode;
		tmp->pre = ptailNode->pre;
		tmp->nxt = ptailNode;
		tmp->key = key;
		tmp->value = value;

		ptailNode->pre->nxt = tmp;
		ptailNode->pre = tmp;

		hashtable[key] = tmp;
		current++;
	}

	~CLRUpool() {
		CDoubleListNode * p = pheadNode->nxt;
		for (; p != NULL; p = p->nxt)
		{
			delete p->pre;
		}
		delete ptailNode;
	}
};

测试

int main(void)
{
	CLRUpool mypool(3);

	mypool.find(1106);

	mypool.find(1102);

	mypool.find(1106);

	mypool.find(1101);

	mypool.find(1104);

	return 0;
}

 结果