题解 | 实现二叉树先序，中序和后序遍历

# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None
#
# 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
#
# 
# @param root TreeNode类 the root of binary tree
# @return int整型二维数组
#
class Solution:
    def threeOrders(self , root: TreeNode) -> List[List[int]]:
        # write code here
        res, pre, mid, after = [], [], [], []
        
        def f1(node: TreeNode, lis):
            if node == None:
                return

            lis.append(node.val)
            f1(node.left, lis)
            f1(node.right, lis)

        def f2(node: TreeNode, lis):
            if node == None:
                return None
            f2( node.left, lis)
            lis.append(node.val)
            f2( node.right, lis)

        def f3(node: TreeNode, lis):
            if node == None:
                return None
            f3( node.left, lis)
            f3( node.right, lis)
            lis.append(node.val)
        f1(root, pre)
        f2(root, mid)
        f3(root, after)
        return [pre, mid, after]

一开始把函数给返回了没绷住，要返回列表