五. 二叉树 3 Binary Tree Preorder Tr

思路：先读入该Node， 然后是它的左节点，最后是右节点。所以，这是一个递归。

"""
Definition of TreeNode:
class TreeNode:
    def __init__(self, val):
        self.val = val
        self.left, self.right = None, None
"""


class Solution:
    """
    @param: root: A Tree
    @return: Preorder in ArrayList which contains node values.
    """
    def preorderTraversal(self, root):
        # write your code here
        self.list = []
        def travel(root):
            if root != None:
                self.list.append(root.val)
                travel(root.left)
                travel(root.right)
            else: 
                None
            
        travel(root)
        return self.list