def isSymmetric(self, root):
        if root is None:
            return True
        return self.help(root.left, root.right) 

    def help(self, left, right):
        ’‘’
        終止條件是左子樹和右子樹是葉子節點的時候
        ‘’‘
        if not left and not right:
            return True
        if not left:
            return False
        if not right:
            return False
        if left.val != right.val:
            return False

        return self.help(left.left, right.right) and self.help(right.left, left.right)