validST

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

class Solution:
    def isValidBST(self, root: TreeNode) -> bool:
        if not root:
            return True

        # if left and right node exists ret=True
        ret = bool(root.left) == bool(root.right) and root.right

        #  so if both the node  exists check the base condition and  store  AND of it
        if ret:
            ret = root.left.val < root.val and  (root.right.val > root.val)
        # ret is false and
        elif root.right != root.left:
            #this was added for edge cases
            if root.right:
                return root.right.val == -1
            if root.left:
                return root.left.val == -1
            #===============================
            return False
        else:
            return True

        # now ret is true
        return ret and self.isValidBST(root.left) and self.isValidBST(root.right)