De/Serialize Binary Tree

README.txt

This problem was asked by Google.

Given the root to a binary tree, implement serialize(root), 
which serializes the tree into a string, and deserialize(s), 
which deserializes the string back into the tree.

For example, given the following Node class

class Node:
    def __init__(self, val, left=None, right=None):
        self.val = val
        self.left = left
        self.right = right
        
The following test should pass:

node = Node('root', Node('left', Node('left.left')), Node('right'))
assert deserialize(serialize(node)).left.left.val == 'left.left'

Program.cs

class Program
{
    static void Main(string[] args)
    {
        Node node = new Node("root", new Node("left", new Node("left.left"), null), new Node("right"));
        string serializedNode = node.Serialize();
        var deserializedNode = Node.Deserialize(serializedNode);
        if (deserializedNode.Left.Left.Val != "left.left")
        {
            throw new Exception("incorrect!");
        }
    }
}

class Node
{
    private const string Nil = "null";

    public string Val { get; private set; }
    public Node Left { get; private set; }
    public Node Right { get; private set; }

    public Node(string val)
        : this(val, null, null)
    {

    }

    public Node(string val, Node left, Node right)
    {
        if (string.IsNullOrWhiteSpace(val) || val.Contains(" ") || val == Nil)
        {
            throw new ArgumentException(nameof(val));
        }
        this.Val = val;
        this.Left = left;
        this.Right = right;
    }

    public string Serialize()
    {
        if (string.IsNullOrWhiteSpace(this.Val))
        {
            return Nil;
        }
        return $"{this.Val} {this.Left?.Serialize() ?? Nil} {this.Right?.Serialize() ?? Nil}";
    }

    public static Node Deserialize(string serialized)
    {
        var splitted = serialized.Split(' ');
        var queue = new Queue<string>(splitted);
        return Node.Deserialize(queue);
    }

    private static Node Deserialize(Queue<string> q)
    {
        if (q.Count == 0)
        {
            return null;
        }
        var val = q.Dequeue();
        if (string.IsNullOrWhiteSpace(val) || val == Nil)
        {
            return null;
        }
        return new Node(val, Deserialize(q), Deserialize(q));
    }
}