PreOrder - non recursion

func preOrder(_ root: TreeNode?) -> [Int]{
    guard let root = root else {
        return []
    }
    var stack = [TreeNode]()
    stack.append(root)
    var res = [Int]()
    while stack.count > 0{
        let cur = stack.removeLast()
        if let right = cur.right{
            stack.append(right)
        }
        if let left = cur.left{
            stack.append(left)
        }
        res.append(cur.val)
    }
    return res
}