题目：

题解：

class Solution:
    def maxDepth(self, root: TreeNode) -> int:
        if not root: return 0
        queue, res = [root], 0
        while queue:
            tmp = []
            for node in queue:
                if node.left: tmp.append(node.left)
                if node.right: tmp.append(node.right)
            queue = tmp
            res += 1
        return res