146. LRU Cache

/*
All operations have O(1) time complexity
Space Complexity : O(capacity)
*/
class LRUCache {
    private class Node {
        int key;
        int value;
        Node prev;
        Node next;

        public Node(int k, int v) {
            key = k;
            value = v;
            prev = null;
            next = null;
        }
    }

    HashMap<Integer, Node> map;
    Node head;
    Node tail;
    int maxCapacity;

    public LRUCache(int capacity) {
        map = new HashMap<Integer, Node>();
        head = new Node(0, 0);
        tail = new Node(0, 0);
        head.next = tail;
        tail.prev = head;
        maxCapacity = capacity;
    }

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

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

    private void moveToHead(Node node) {
        deleteNode(node);
        addToHead(node);
    }

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

        Node cur = map.get(key);
        moveToHead(cur);
        return cur.value;
    }

    public void put(int key, int value) {
        if (maxCapacity <= 0) {
            return;
        }

        if (map.containsKey(key)) {
            Node cur = map.get(key);
            cur.value = value;
            moveToHead(cur);
            return;
        }

        if (map.size() == maxCapacity) {
            map.remove(tail.prev.key);
            deleteNode(tail.prev);
        }

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

/**
 * 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);
 */