Swift二叉树排序 递归前中后序

public class TreeNode {
    public var val: Int
    public var left: TreeNode?
    public var right: TreeNode?
    public init(_ val: Int) {
        self.val = val
        self.left = nil
        self.right = nil
    }
}

class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view.
        
        let Tree1 = TreeNode.init(1)
        let Tree2 = TreeNode.init(2)
        let Tree3 = TreeNode.init(3)
        let Tree4 = TreeNode.init(4)
        let Tree5 = TreeNode.init(5)
        let Tree6 = TreeNode.init(6)
        let Tree7 = TreeNode.init(7)
        let Tree8 = TreeNode.init(8)
        let Tree9 = TreeNode.init(9)
        let Tree10 = TreeNode.init(10)
        let Tree11 = TreeNode.init(11)
        
        Tree1.left = Tree2
        Tree1.right = Tree3
        Tree2.left = Tree4
        Tree3.left = Tree5
        Tree3.right = Tree6
        Tree4.right = Tree7
        Tree5.left = Tree8
        Tree5.right = Tree9
        Tree6.right = Tree10
        Tree7.left = Tree11
        
        print(preorderTraversal(Tree1))
        
    }
    
    ///二叉树的前序 中序 后序排序
    func preorderTraversal(_ root: TreeNode?) -> [Int] {
        var array = [Int]();
        if root == nil {
            return array;
        }
        //print(root?.val as Any)//前序
        array.append(contentsOf: preorderTraversal(root?.left));
        //print(root?.val as Any)//中序
        array.append(contentsOf: preorderTraversal(root?.right));
        //print(root?.val as Any)//后序
        return array;
    }
}