[数据结构与算法]146. LRU 缓存机制（C++实现）

struct DLinkedNode{
    int key;
    int value;
    DLinkedNode *next;
    DLinkedNode *prev;
    DLinkedNode():key(0), value(0), next(nullptr), prev(nullptr){}
    DLinkedNode(int _key, int _value):key(_key), value(_value), next(nullptr), prev(nullptr){};
};

class LRUCache {
private:
    int _capacity; //记录双向链表容量
    DLinkedNode *dummyHead;
    DLinkedNode *dummyTail;
    int size; //记录当前有多少个元素
    unordered_map<int, DLinkedNode*> map;
    
public:
    
    //构造函数
    LRUCache(int capacity): _capacity(capacity), size(0){
        dummyHead = new DLinkedNode();
        dummyTail = new DLinkedNode();
        dummyTail->prev = dummyHead;
        dummyHead->next = dummyTail;
    }
    
    int get(int key) {
        //找不到
        if(map.count(key)==0)
            return -1;
        //找到，调整到链表头部
        DLinkedNode *node = map[key];
        moveToHead(node);
        return node->value;
    }
    
    void put(int key, int value) {
        //已经存在，更新值，移至链表头部
        if(map.count(key)==1){
            DLinkedNode *node = map[key];
            node->value = value;
            moveToHead(node);
        }
        
        //不存在：1.没到容量，直接插入；2.已满，先删除，再插入
        else{
            DLinkedNode *node = new DLinkedNode();
            node->key = key;
            node->value = value;
            map[key] = node;
            
            size++;
            //还没满，可以直接插入
            if(size<=_capacity){
                addToHead(node);
            }
            //满了，先删除，再插入，释放删除节点所占内存
            else{
                DLinkedNode *removeNode = removeTail();
                map.erase(removeNode->key);
                addToHead(node);
                delete removeNode;
                size--;
            }
            
        }
        
        
        
    }
    void addToHead(DLinkedNode *node){
        node->next = dummyHead->next;
        node->prev = dummyHead;
        dummyHead->next->prev = node;
        dummyHead->next = node;
    }
    
    void removeNode(DLinkedNode *node){
        node->prev->next = node->next;
        node->next->prev = node->prev;
    }
    
    void moveToHead(DLinkedNode *node){
        removeNode(node);
        addToHead(node);
    }
    
    //删除尾部节点，但是同时要删除哈希表中的节点，所以要返回值
    DLinkedNode *removeTail(){
        DLinkedNode *node = dummyTail->prev;
        removeNode(node);
        return node;
    }
};


/**
 * 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);
 */