思路

考虑从递归出发，联想递归三部曲：返回什么、传入的参数是什么、遍历的方式是什么。此题现在需要我们整个树，并且需要从根节点出发，因此我们选择先序遍历即可。另一张办法，则是选择通过队列实现层次遍历，统计队列长度也就是树的最大深度

# 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 maxDepth(self, root: Optional[TreeNode]) -> int:#递归def preorder(node):if node==None:return 0left=preorder(node.left)right=preorder(node.right)return 1+max(left,right)return preorder(root)

# 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 maxDepth(self, root: Optional[TreeNode]) -> int:que=deque()if root:que.append(root)result=[]while que:L=len(que)level=[]while L:L-=1node=que.popleft()if node:level.append(node.val)if node.left:que.append(node.left)if node.right:que.append(node.right)result.append(level)return len(result)