[数据结构与算法]实现二叉树先序，中序和后序遍历

二叉树先序：根左右
二叉树中序：左根右
二叉树后序：左右根
代码：
# class TreeNode:
# def init(self, x):
# self.val = x
# self.left = None
# self.right = None

# @param root TreeNode类 the root of binary tree
# @return int整型二维数组

class Solution:
    def threeOrders(self , root ):
        # write code here
        pre_list,mid_list,aft_list=[],[],[]
        def preorder(root):
            if not root:
                return None
            pre_list.append(root.val)
            preorder(root.left)
            preorder(root.right)
        def midorder(root):
            if not root:
                return None
            midorder(root.left)
            mid_list.append(root.val)
            midorder(root.right)
        def aftorder(root):
            if not root:
                return None
            aftorder(root.left)
            aftorder(root.right)
            aft_list.append(root.val)
        preorder(root)
        midorder(root)
        aftorder(root)
        return [pre_list,mid_list,aft_list]