LRU.java

public class LRUCache {
    class DLinkedNode {
        int key;
        int value;
        DLinkedNode prev;
        DLinkedNode next;
    }

    private DLinkedNode head, tail;
    private HashMap<Integer, DLinkedNode> cache = new HashMap<>();
    private int size;
    private int capacity;

    private void addNode(DLinkedNode node) {
        /*always add node right after head*/
        node.prev = head;
        node.next = head.next;
        head.next = node;
        node.next.prev = node;
    }

    private void removeNode(DLinkedNode node) {
        /*remove an existing node*/
        DLinkedNode prev = node.prev;
        DLinkedNode next = node.next;
        prev.next = next;
        next.prev = prev;
    }

    private void moveToHead(DLinkedNode node) {
        removeNode(node);
        addNode(node);
    }

    private DLinkedNode popTail() {
        DLinkedNode res = tail.prev;
        removeNode(res);
        return res;
    }

    public LRUCache(int capacity) {
        this.size = 0;
        this.capacity = capacity;
        head = new DLinkedNode();
        tail = new DLinkedNode();
        head.next = tail;
        tail.prev = head;
    }

    public int get(int key) {
        DLinkedNode node = cache.get(key);
        if (node == null) {
            return -1;
        }
        moveToHead(node);
        return node.value;
    }

    public void put(int key, int value) {
        DLinkedNode node = cache.get(key);

        if (node == null) {
            DLinkedNode newNode = new DLinkedNode();
            newNode.key = key;
            newNode.value = value;

            cache.put(key, newNode);
            addNode(newNode);
            size++;
            if (size > capacity) {
                DLinkedNode tail = popTail();
                cache.remove(tail.key);
                --size;
            }
        } else {
            node.value = value;
            moveToHead(node);
        }
    }


}