Solution.cpp

struct Node {
    int k;
    int v;
    Node* prev;
    Node* next;

    Node(): k(0), v(0), prev(nullptr), next(nullptr) {}
    Node(int key, int val): k(key), v(val), prev(nullptr), next(nullptr) {}
};

class LRUCache {
public:
    LRUCache(int capacity): cap(capacity), size(0) {
        head = new Node();
        tail = new Node();
        head->next = tail;
        tail->prev = head;
    }
    
    int get(int key) {
        if (!cache.count(key)) return -1;
        Node* node = cache[key];
        moveToHead(node);
        return node->v;
    }
    
    void put(int key, int value) {
        if (cache.count(key))
        {
            Node* node = cache[key];
            node->v = value;
            moveToHead(node);
        }
        else
        {
            Node* node = new Node(key, value);
            cache[key] = node;
            addToHead(node);
            ++size;
            if (size > cap)
            {
                node = removeTail();
                cache.erase(node->k);
                --size;
            }
        }
    }

private:
    unordered_map<int, Node*> cache;
    Node* head;
    Node* tail;
    int cap;
    int size;

    void moveToHead(Node* node) {
        removeNode(node);
        addToHead(node);
    }

    void removeNode(Node* node) {
        node->prev->next = node->next;
        node->next->prev = node->prev;
    }

    void addToHead(Node* node) {
        node->next = head->next;
        node->prev = head;
        head->next = node;
        node->next->prev = node;
    }

    Node* removeTail() {
        Node* node = tail->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);
 */