104.二叉树的最大深度

难度：简单
题目描述：

思路总结：思路尼玛，多来点这种题，好有时间做剑指Offer。PS:这套模板实际上就是一个前序遍历的模板，思路很有限。
题解一：（递归）

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    def maxDepth(self, root: TreeNode) -> int:
        if not root:return 0
        res = 0
        def helper(node, level):
            if not node.left and not node.right:
                nonlocal res
                res = max(level, res)
            if node.left:
                helper(node.left, level+1)
            if node.right:
                helper(node.right, level+1)
        helper(root, 1)
        return res

题解一结果：
题解二：（迭代）

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    def maxDepth(self, root: TreeNode) -> int:
        if not root:return 0
        res = 0
        stack = [(root, 1)]
        while stack:
            cur, level = stack.pop()
            if not cur.left and not cur.right:
                res = max(res, level)
            if cur.left:
                stack.append((cur.left, level+1))
            if cur.right:
                stack.append((cur.right, level+1))
        return res

题解二结果：