递归

# 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
        return [self.front(root), self.middle(root), self.back(root)]
    
    def front(self, root, var=None):
        if root is None:
            return    # 终止递归
        if var is None:
            var = []    # 开始递归，初始化结果存储列表
        var.append(root.val)
        lt = self.front(root.left, var)
        rt = self.front(root.right, var)
        return var
        
    
    def middle(self, root, var=None):
        if root is None:
            return
        if var is None:
            var = []
        lt = self.middle(root.left, var)
        var.append(root.val)
        rt = self.middle(root.right, var)
        return var
    
    def back(self, root, var=None):
        if root is None:
            return
        if var is None:
            var = []
        lt = self.back(root.left, var)
        rt = self.back(root.right, var)
        var.append(root.val)
        return var