class Solution:
    def helper(self, node):
      if node == None:
        return 0
      left_depth = 1 + self.helper(node.left)
      right_depth = 1 + self.helper(node.right)
      return max(left_depth,right_depth)

    def maxDepth(self, root: TreeNode) -> int:
      return self.helper(root)