104. maximum depth of binary tree

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution(object):
    def maxDepth(self, root):
        """
        :type root: TreeNode
        :rtype: int
        """
        return self.depth(root, layer=0)

    def depth(self, root, layer):
        if root == None:
            return layer
        layer += 1
        return max(self.depth(root.left, layer), self.depth(root.right, layer))