class Solution:
    def isValidBST(self, root: Optional[TreeNode]) -> bool:

        return self.helper(root, -math.inf, math.inf)


    def helper(self, root, minVal, maxVal):

        if not root:
            return True

        # ensure the root is in the prescribed range
        if not (minVal < root.val < maxVal):
            return False

        # check condition for left subtree
        left = self.helper(root.left, minVal, root.val)

        # check condition for right subtree
        right = self.helper(root.right, root.val, maxVal)

        # return if both the subtree condition are satisfied
        return left and right