func levelOrder(root: Node?) -> Void {
        guard let root = root else { return }
        var result: [Int] = []
        var queue:[Node] = []
        queue.append(root)
        while !queue.isEmpty {
            let node = queue.removeFirst()
            if let left = node.left {
                queue.append(left)
            }
            
            if let right = node.right {
                queue.append(right)
            }
            result.append(node.data)
        }
        print(result.map { String($0) }.joined(separator: " "))
    	// Complete the function
    } // End of levelOrder function