[LeetCode]LRU Cache

最新推荐文章于 2025-05-03 16:15:14 发布

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本文介绍了一种使用双向链表和哈希表实现LRU(Least Recently Used)缓存策略的方法。该方法能够高效地在O(1)时间内完成获取(get)和设置(set)操作。通过维护一个双向链表来跟踪最近使用的缓存项，并利用哈希表快速定位链表中的节点。

class LRUCache{
    struct ListNode//for LRU
    {
        int value;
        int key;
        ListNode* prev;
        ListNode* next;
        ListNode(int _key, int _value):key(_key), value(_value), prev(NULL), next(NULL){}
    };
public:
    LRUCache(int capacity) {
        m_MaxCapacity = capacity;
        m_head = NULL;
        m_tail = NULL;
        m_hashMap.clear();
    }
    
    int get(int key) {
        auto it = m_hashMap.find(key);
        if(it != m_hashMap.end())
        {
            move2Head(it->second);
            return it->second->value;
        }
        else return -1;
    }
    
    void set(int key, int value) {
        auto it = m_hashMap.find(key);
        if(it != m_hashMap.end())//existed
        {
            ListNode* pCurNode = it->second;
            pCurNode->value = value;
            //attune current node before the head
            move2Head(pCurNode);
        }
        else//not existed
        {
            if((int)m_hashMap.size() == m_MaxCapacity)
            {
                //first invalidate the least recently used item, e.g, delete the tail
                deleteTail();
            }
            //insert before the head
            insert2Head(key, value);
        }
    }
private:
    void move2Head(ListNode* pCurNode)
    {
        if(pCurNode != m_head)
        {
            if(pCurNode == m_tail) m_tail = pCurNode->prev;
            pCurNode->prev->next = pCurNode->next;
            if(pCurNode->next != NULL) pCurNode->next->prev = pCurNode->prev;
            pCurNode->prev = NULL;
            m_head->prev = pCurNode;
            pCurNode->next = m_head;
            m_head = pCurNode;
        }
    }
    void deleteTail()
    {
        if(m_tail != NULL)
        {
            int deleteKey = m_tail->key;
            auto it = m_hashMap.find(deleteKey);
            m_hashMap.erase(it);
            ListNode* pDeleteNode = m_tail;
            if(m_tail->prev != NULL) m_tail->prev->next = NULL;
            else m_head = NULL;
            m_tail = m_tail->prev;
            delete pDeleteNode;
        }
        //else input capacity error
    }
    void insert2Head(int key, int value)
    {
        ListNode* pNewNode = new ListNode(key, value);
        if(m_head != NULL) 
        {
            m_head->prev = pNewNode;
            pNewNode->prev = NULL;
            pNewNode->next = m_head;
            m_head = pNewNode;
        }
        else m_head = m_tail = pNewNode;
        m_hashMap.insert(make_pair(key, pNewNode));
    }
private:
    unordered_map<int, ListNode*> m_hashMap;
    ListNode* m_head;
    ListNode* m_tail;
    int m_MaxCapacity;
};