text: # Given a binary tree, find its maximum depth.
  # #
  # # The maximum depth is the number of nodes along the longest path
  # # from the root node down to the farthest leaf node.
  # #
  # # Note: A leaf is a node with no children.
  # #
  # # Example:
  # #
  # # Given binary tree [3,9,20,null,null,15,7],
  # #
  # #     3
  # #    / \
  # #   9  20
  # #     /  \
  # #    15   7
  # # return its depth = 3.
  
  
  class TreeNode:
      def __init__(self, x):
          self.val = x
          self.left = None
          self.right = None
  
  
  class Solution:
      def maxDepth(self, root):
          if not root:
              return 0
  
          return 1 + max(self.maxDepth(root.left), self.maxDepth(root.right))
  