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.

HashMap + Doubly LinkedList

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public class LRUCache {
    int capacity;
    HashMap<Integer, Node> cache;
    Node head;
    Node tail;
    
    public LRUCache(int capacity) {
        this.capacity = capacity;
        cache = new HashMap<>();
        head = new Node(0, 0);
        tail = new Node(0, 0);
        head.prev = null;
        head.next = tail;
        tail.next = null;
        tail.prev = head;
    }
    
    public int get(int key) {
        Node node = cache.get(key);
        if (node != null) {
            deleteNode(node);
            addToHead(node);
            return node.value;
        }
        return -1;
    }
    
    public void set(int key, int value) {
        Node existing = cache.get(key);
        if (existing != null) {
            deleteNode(existing);
            existing.value = value;
            addToHead(existing);
        } else {
            if (cache.size() >= capacity) {
                cache.remove(tail.prev.key);
                deleteNode(tail.prev);
            }
            Node node = new Node(key, value);
            cache.put(key, node);
            addToHead(node);
        }
    }
    
    public void deleteNode(Node node){
        node.prev.next = node.next;
        node.next.prev = node.prev;
    }
    
    public void addToHead(Node node) {
        node.next = head.next;
        node.next.prev = node;
        head.next = node;
        node.prev = head;
    }
    
    public class Node {
        int key;
        int value;
        Node prev;
        Node next;
        
        public Node(int key, int value) {
            this.key = key;
            this.value = value;
        }
    }
}

LinkedHashMap

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public class LRUCache {
    Map<Integer, Integer> map;
    
    public LRUCache(int capacity) {
        map = new LinkedHashMap<Integer, Integer>(16, 0.75f, true){
            protected boolean removeEldestEntry(Map.Entry eldest) {
                return size() > capacity;
            }
        };
    }
    
    public int get(int key) {
        return map.getOrDefault(key, -1);
    }
    
    public void set(int key, int value) {
        map.put(key, value);
    }
}