二叉搜索树中的每个结点，其左子树的所有结点值都小于该结点值，右子树的所有结点值都大于该结点值。以下代码对给定的整数数组(假设数组中没有数值相等的元素)，构造一个对应的二叉搜索树，横线上应填写（    ）:

class TreeNode:

      def __init__(self, x):

            self.val = x

            self.left = None

            self.right = None

class Solution:

      def isValidBST(self, root: TreeNode) -> bool:

            def helper(node, min_val, max_val):

                  if not node:

                        return True

                  ————————————

                        return False

                  return helper(node.left, min_val, node.val) and helper(node.right,

node.val, max_val)

              return helper(root, float('-inf'), float('inf'))