146-Lru-Cache.cpp

/*
    Design data structure that follows constraints of an LRU cache

    Hash map + doubly linked list, left = LRU, right = MRU
    get: update to MRU, put: update to MRU, remove LRU if full

    Time: O(1)
    Space: O(capacity)
*/

class Node {
public:
    int k;
    int val;
    Node* prev;
    Node* next;
    
    Node(int key, int value) {
        k = key;
        val = value;
        prev = NULL;
        next = NULL;
    }
};

class LRUCache {
public:
    LRUCache(int capacity) {
        cap = capacity;
        
        left = new Node(0, 0);
        right = new Node(0, 0);
        
        left->next = right;
        right->prev = left;
    }
    
    int get(int key) {
        if (cache.find(key) != cache.end()) {
            remove(cache[key]);
            insert(cache[key]);
            return cache[key]->val;
        }
        return -1;
    }
    
    void put(int key, int value) {
        if (cache.find(key) != cache.end()) {
            remove(cache[key]);
            
            // Free allocated memory for the removed node
            delete cache[key];
        }
        cache[key] = new Node(key, value);
        insert(cache[key]);
        
        if (cache.size() > cap) {
            // remove from list & delete LRU from map
            Node* lru = left->next;
            remove(lru);
            cache.erase(lru->k);
            
            // Free allocated memory for the removed node
            delete lru;
        }
    }
private:
    int cap;
    unordered_map<int, Node*> cache; // {key -> node}
    Node* left;
    Node* right;
    
    // remove node from list
    void remove(Node* node) {
        Node* prev = node->prev;
        Node* next = node->next;
        
        prev->next = next;
        next->prev = prev;
    }
    
    // insert node at right
    void insert(Node* node) {
        Node* prev = right->prev;
        Node* next = right;
        
        prev->next = node;
        next->prev = node;
        
        node->prev = prev;
        node->next = next;
    }
};

/**
 * 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);
 */