https://leetcode-cn.com/problems/lru-cache/

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struct node {
    int key, value;
    node * next;
    node * pre;
    node (int k, int v) {
        key = k;
        value = v;
        next = pre = NULL;
    }
    node () {
        key = value = 0;
        next = pre = NULL;
    }
};

class LRUCache {
    unordered_map<int, node *>mp;
    node * head;
    node * tail;
    int size;
    int capacity;
public:
    LRUCache(int _capacity) {
        capacity = _capacity;
        size = 0;
        head = new node();
        tail = new node();
        head -> next = tail;
        tail -> pre = head;
    }
    
    int get(int key) {
        if(!mp.count(key)) {
            return -1;
        }
        node * p = mp[key];
        node * tmp1 = p -> pre;
        node * tmp2 = p -> next;
        tmp1 -> next = tmp2;
        tmp2 -> pre = tmp1;
        
        node * tmp3 = tail -> pre;
        tmp3 -> next = p;
        p -> next = tail;
        tail -> pre = p;
        p -> pre = tmp3;

        return p -> value;
    }
    
    void put(int key, int value) {
        if(!mp.count(key)) {
            node * tmp = new node(key, value);
            mp[key] = tmp;
            size++;

            node * tmp2 = tail -> pre;
            tmp2 -> next = tmp;
            tmp -> next = tail;
            tail -> pre = tmp;
            tmp -> pre = tmp2;
            if(size > capacity) {
                node * p = head -> next;
                head -> next = p -> next;
                p -> next -> pre = head;
                mp.erase(p -> key);
                delete p;
                size--;
            }
        } else {
            node * p = mp[key];
            p -> value = value;
            node * tmp1 = p -> pre;
            node * tmp2 = p -> next;
            tmp1 -> next = tmp2;
            tmp2 -> pre = tmp1;
            
            node * tmp3 = tail -> pre;
            tmp3 -> next = p;
            p -> next = tail;
            tail -> pre = p;
            p -> pre = tmp3;
        }
    }
};

/**
 * 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);
 */