We use DFS to recursivly get the depth of the binary tree.
# 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 root is None:
return 0
return max(self.maxDepth(root.left), self.maxDepth(root.right)) + 1