题目描述：

第一次提交：

class Solution(object):
    def maxDepth(self, root):
        """
        :type root: TreeNode
        :rtype: int
        """
        if not root:
            return 0
        elif root.left==None and root.right==None:
            return 1
        elif root.left==None and root.right!=None:
            return 1+self.maxDepth(root.right)
        elif root.left!=None and root.right==None:
            return 1+self.maxDepth(root.left)
        else:
            return 1+max(self.maxDepth(root.left),self.maxDepth(root.right))

优化后：

class Solution(object):
    def maxDepth(self, root):
        """
        :type root: TreeNode
        :rtype: int
        """
        if root!=None:
            return max(self.maxDepth(root.left), self.maxDepth(root.right)) + 1
        else:
            return 0