day13 代码随想录算法训练营递归遍历-526互联

题目：

我的感悟：

用helper内部函数写更好

理解难点：

代码难点：

代码示例：

前序

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def preorderTraversal(self, root: Optional[TreeNode]) -> List[int]:
        res = []
        def helper(root):
            if not root:return 
            res.append(root.val)
            helper(root.left)
            helper(root.right)
        helper(root)
        return res

中序

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def preorderTraversal(self, root: Optional[TreeNode]) -> List[int]:
        res = []
        def helper(root):
            if not root:return             
            helper(root.left)
            res.append(root.val)    # 结果放在这里,中序
            helper(root.right)
        helper(root)
        return res

后序

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def preorderTraversal(self, root: Optional[TreeNode]) -> List[int]:
        res = []
        def helper(root):
            if not root:return             
            helper(root.left)             
            helper(root.right)
            res.append(root.val)  # 结果放在后面,后序 
        helper(root)
        return res