LRU Cache -- LeetCode

Design and implement a data structure for Least Recently Used (LRU) cache. It should support the following operations: get and set.

get(key) - Get the value (will always be positive) of the key if the key exists in the cache, otherwise return -1.
set(key, value) - Set or insert the value if the key is not already present. When the cache reached its capacity, it should invalidate the least recently used item before inserting a new item.

思路：使用哈希表来记录key与值。为了实现LRU，我们需要实现一个双向链表，最少使用的在链表头，最近使用的在链表尾。对于每一个key，我们还需要一个哈希表来记录它在该链表中的地址。

在代码实现过程中，新申请一个链表只能用new的形式来申请，该语句将返回一个申请后的指针。

class LRUCache{
public:
    class dlist
    {
        public:
            dlist *pre, *next;
            int key;
            dlist(int d, dlist *p, dlist *f) : key(d), pre(p), next(f){}
    };
    int cap;
    dlist *front, *tail;
    unordered_map<int, int> data;
    unordered_map<int, dlist *> addr;
    LRUCache(int capacity) {
        cap = capacity;
        front = new dlist(0, NULL, NULL);
        tail = new dlist(0, NULL, NULL);
        front->next = tail;
        tail->pre = front;
    }
    
    int get(int key) {
        if (data.count(key) == 0) return -1;
        Delete(addr[key]);
        AddtoTail(addr[key]);
        return data[key];
    }
    void AddtoTail(dlist *node)
    {
        tail->pre->next = node;
        node->pre = tail->pre;
        node->next = tail;
        tail->pre = node;
    }
    void Delete(dlist *node)
    {
        node->pre->next = node->next;
        node->next->pre = node->pre;
    }
    void set(int key, int value) {
        if (data.size() == cap && data.count(key) == 0)
        {
            if (front->next == tail) return;
            data.erase(front->next->key);
            addr.erase(front->next->key);
            Delete(front->next);
        }
        if (data.count(key) == 0)
        {
            data.insert(make_pair(key, value));
            dlist *ent = new dlist(key, NULL, NULL);
            addr.insert(make_pair(key, ent));
            AddtoTail(ent);
        }
        else
        {
            data[key] = value;
            Delete(addr[key]);
            AddtoTail(addr[key]);
        }
    }
};