lruLLCache.java

class LRUCache {

    class Node {
        int key, val;
        Node prev, next;

        Node(int k, int v) {
            key = k;
            val = v;
        }
    }

    private Map<Integer, Node> map;
    private int capacity;
    private Node head, tail;

    public LRUCache(int capacity) {
        this.capacity = capacity;
        map = new HashMap<>();

        head = new Node(0, 0);
        tail = new Node(0, 0);

        head.next = tail;
        tail.prev = head;
    }

    public int get(int key) {
        if (!map.containsKey(key)) return -1;

        Node node = map.get(key);
        remove(node);
        insert(node);

        return node.val;
    }

    public void put(int key, int value) {
        if (map.containsKey(key)) {
            remove(map.get(key));
        }

        Node node = new Node(key, value);
        insert(node);
        map.put(key, node);

        if (map.size() > capacity) {
            Node lru = tail.prev;
            remove(lru);
            map.remove(lru.key);
        }
    }

    private void remove(Node node) {
        node.prev.next = node.next;
        node.next.prev = node.prev;
    }

    private void insert(Node node) {
        node.next = head.next;
        node.prev = head;

        head.next.prev = node;
        head.next = node;
    }
}