146. LRU Cache

struct Node {
    int key;
    int val;
    Node* next;
    Node* prev;
    Node(int key, int val) : key(key), val(val), next(nullptr), prev(nullptr) {}
};

class LRUCache {
private:
    int cap;

    unordered_map<int, Node*> m;

    Node* head = new Node(-1, -1);
    Node* tail = new Node(-1, -1);

    // head <-> 1 <-> 2 <-> 3 <-> tail
    //                ^ node
    //          ^ prev_node
    //                      ^ next_node
    void remove(Node* node)
    {
        Node* prev_node = node->prev;
        Node* next_node = node->next;

        prev_node->next = next_node;
        next_node->prev = prev_node;
    }

    // head <-> 1 <-> 2 <-> tail
    //                ^ prev_node
    //                       ^ next_node
    void add(Node* node)
    {
        Node* prev_node = tail->prev;
        Node* next_node = tail;

        prev_node->next = node;
        next_node->prev = node;

        node->next = next_node;
        node->prev = prev_node;
    }

public:
    LRUCache(int capacity)
    {
        cap = capacity;
        // head <-> tail
        head->next = tail;
        tail->prev = head;
    }

    ~LRUCache()
    {
        Node* cur = head;
        while (cur) {
            Node* next = cur->next;
            delete cur;
            cur = next;
        }
    }

    int get(int key)
    {
        if(!m.count(key)) return -1;
        Node* node_to_get = m[key];
        remove(node_to_get);
        add(node_to_get);
        return node_to_get->val;
    }

    void put(int key, int value)
    {
        if(m.count(key)) remove(m[key]);

        Node* node_to_put = new Node(key, value);
        m[key] = node_to_put;
        add(node_to_put);

        if (m.size() > cap)
        {
            Node* node_to_delete = head->next;
            remove(node_to_delete);
            m.erase(node_to_delete->key);
            delete node_to_delete;
        }
    }
};

/**
 * 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);
 */

• T: $O(1)$
• S: $O(capacity)$