解法一：递归

  1. class Solution:
  2.     def maxDepth(self, root: TreeNode) -> int:
  3.         if not root:
  4.             return 0
  5.         left = self.maxDepth(root.left) + 1
  6.         right = self.maxDepth(root.right) + 1
  7.         return max(left, right)

TODO：可以尝试改造成尾递归，即在递归参数中将depth作为参数传入。

解法二：带层控制的BFS

  1. class Solution:
  2.     def maxDepth(self, root: TreeNode) -> int:
  3.         queue = [root] if root else []
  4.         ans = 0
  5.         while queue:
  6.             ans += 1
  7.             for _ in range(len(queue)):
  8.                 node = queue.pop(0)
  9.                 if node.left:
  10.                     queue.append(node.left)
  11.                 if node.right:
  12.                     queue.append(node.right)
  13.         return ans