19. 二叉树的镜像

输入树：
      8
     / 
    6  10
   /  / 
  5  7 9 11

 [8,6,10,5,7,9,11,null,null,null,null,null,null,null,null] 
输出树：
      8
     / 
    10  6
   /  / 
  11 9 7  5

 [8,10,6,11,9,7,5,null,null,null,null,null,null,null,null]

# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class :
    # 返回镜像树的根节点
    def Mirror(self, root):
        # write code here
        if not root:
            return root
        root.left,root.right = root.right,root.left
        self.Mirror(root.left)
        self.Mirror(root.right)
        return root