LRU Cache

Hard LRU Cache

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.

 
class DllNode
{
public:
    DllNode *prev;
    DllNode *next;
    int val;
    int key;

    DllNode(int key, int val)
    {
        this->key = key;
        this->val = val;
        this->prev = this;
        this->next = this;
    }
    
    void Detach()
    {
        prev->next = next;
        next->prev = prev;
    } 
    
    void AddLeft(DllNode *node)
    {
       prev->next = node;
       node->prev = prev;
       node->next = this;
       prev = node;
    }
};

class LRUCache{
public:
    int m_cap;
    DllNode m_head;
    unordered_map<int, DllNode *> m_map;
   
    LRUCache(int capacity): m_head(0,0)
    {
        m_cap = capacity;
    }
    
    int get(int key) {
        if (m_map.count(key) ==0)
        return -1;
        DllNode *node = m_map[key];
        node->Detach();
        m_head.AddLeft(node);
        return node->val;
    }
    
    void set(int key, int value) {
        
        if (m_map.find(key) != m_map.end())
        {
            DllNode *node = m_map[key];
            node->Detach();
            delete node;
            m_map.erase(key);
        }
        
        if (m_map.size() == m_cap)
        {
           DllNode *oldest = m_head.next;
           oldest->Detach();
           m_map.erase(oldest->key);
           delete oldest;
        }
        DllNode *node = new DllNode(key, value);
        m_head.AddLeft(node);
        m_map[key] = node;
    }
};