Leetcode146 LRU缓存机制（哈希表+双向链表）_leedcode 146. lru 缓存机制(哈希+双向链表)

$O(1)$ 时间复杂度可以通过哈希表+双向链表来实现
哈希表存储 <key, node ptr>，可以在 $O(1)$ 时间内找到对应key值的节点。
双向链表节点内存储{key，value，prev，next}四个属性。每个节点的插入、删除操作都可以在常数时间完成。
链表中有固定的head, tail节点，最近访问的节点移动到tail之前，最不常访问key值对应的节点则位于head之后。
因此，每次访问只需要改变节点的位置，完成常数时间的更新。

附上代码：

struct BiLinkNode{
    int key, value;
    BiLinkNode* prev;
    BiLinkNode* next;
    BiLinkNode(): key(0), value(0), prev(nullptr), next(nullptr) {}
    BiLinkNode(int _key, int _value, BiLinkNode* _prev, BiLinkNode* _next): key(_key), value(_value), prev(_prev), next(_next) {}
    BiLinkNode(int _key, int _value): key(_key), value(_value), prev(nullptr), next(nullptr) {}
};
class LRUCache {
    map<int, BiLinkNode*> cache;
    int capacity;
    BiLinkNode* head = new BiLinkNode(-1,-1);
    BiLinkNode* tail = new BiLinkNode(-1,-1);
public:
    LRUCache(int capacity) {
        this->capacity = capacity;
        head->next = tail;
        head->prev = head;
        tail->prev = head;
        tail->next = tail;
    }
    
    int get(int key) {
        if(!cache.count(key)) return -1;
        cache[key]->prev->next = cache[key]->next;
        cache[key]->next->prev = cache[key]->prev;
        cache[key]->next = tail;
        cache[key]->prev = tail->prev;
        tail->prev->next = cache[key];
        tail->prev = cache[key];
        return cache[key]->value;
    }
    
    void put(int key, int value) {
        if(cache.count(key)) {
            cache[key]->value = value;
            cache[key]->next->prev = cache[key]->prev;
            cache[key]->prev->next = cache[key]->next;
        }
        else{
            if(capacity <= cache.size()){
                int del = head->next->key;
                cache[del]->next->prev = cache[del]->prev;
                cache[del]->prev->next = cache[del]->next;
                cache.erase(del);
            }
            cache[key] = new BiLinkNode(key, value);
        }
        cache[key]->prev = tail->prev;
        cache[key]->next = tail;
        tail->prev = cache[key];
        cache[key]->prev->next = cache[key];
    }
};

/**
 * 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);
 */
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