分而治之

return max( 头结点左子树的最大深度, 头结点右子树的最大深度)+1

# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None
# 
# @param pRoot TreeNode类 
# @return int整型
class Solution:
    def TreeDepth(self , pRoot: TreeNode) -> int:
        # write code here
        if not pRoot:
            return 0
        return max(self.TreeDepth(pRoot.left),self.TreeDepth(pRoot.right))+1