# Definition for a binary tree node.
class TreeNode(object):
    def __init__(self, x):
         self.val = x
         self.left = None
         self.right = None

class Solution():
    def isValidBST(self, root):
        if root.val is None:
            return [float("inf"), -float("inf")]
        l = self.isValidBST(root.left)
        r = self.isValidBST(root.right)
        if not l or not r:
            return False
        if l[1] < root.val < r[0]:
            return [min(l[0], root.val), max(r[1], root.val)]