98_validate_binary_search_tree

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

class Solution(object):
    def isValidBST(self, root):
        """
        :type root: TreeNode
        :rtype: bool
        """
        if not root:
            return True
        
        def isBSTHelper(node, lower_limit, upper_limit):
            if lower_limit is not None and node.val <= lower_limit:
                return False
            if upper_limit is not None and upper_limit <= node.val:
                return False
            
            left = isBSTHelper(node.left, lower_limit, node.val) if node.left else True
            if left:
                right = isBSTHelper(node.right, node.val, upper_limit) if node.right else True
                return right
            else:
                return False
            
        return isBSTHelper(root, None, None)


class Solution(object):
    def isValidBST(self, root, lessThan = float('inf'), largerThan = float('-inf')):
        if not root:
            return True
        if root.val <= largerThan or root.val >= lessThan:
            return False
        return self.isValidBST(root.left, min(lessThan, root.val), largerThan) and \
               self.isValidBST(root.right, lessThan, max(root.val, largerThan))