二叉树的最大深度和最小深度

 思路：采用二叉树的深度优先遍历，如果是叶子节点更新最大值和最小值。

# 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
        """
        if not root: return 0
        self.max=0
        self._dfs(root,0)
        return self.max+1
    def _dfs(self,node,level):
        if not node: return 
        if node.left==None and node.right==None:
            if level>self.max:
                self.max=level
            return 
        self._dfs(node.left,level+1)
        self._dfs(node.right,level+1)
        return