def isValidBST(self, root):
        """
        Google Phone Interview Question, 20 Sep 2013
        recursive dfs

        alternative answer: convert the tree the array and judge whether it is sorted
        :param root: a tree node
        :return: boolean
        """
        if not root:
            return True

        if not self.isValidBST(root.left):
            return False
        if not self.isValidBST(root.right):
            return False

        if root.left:
            if not self.get_largest(root.left) < root.val:
                return False
        if root.right:
            if not self.get_smallest(root.right) < root.val:
                return False


        return True
dataset/ETHPy150Open algorhythms/LeetCode/098 Validate Binary Search Tree.py/Solution.isValidBST/swapped operands of `<`