设二叉树共有n个结点，给定以下递归实现的前序遍历函数：

class TreeNode:
    def __init__(self, x):
        self.val = x
        self.left = None
        self.right = None


def preorder(root, res):
    if root is None:
        return
    res.append(root.val)
    preorder(root.left, res)
    preorder(root.right, res)


def preorderTraversal(root):
    res = []
    preorder(root, res)
    return res