Validate Binary Search Tree

def isValidBST(root):
    def valid(node, left, right):
        if not node:
            return True
        if not (node.val < right and node.val > left):
            return False
            
        return (valid(node.left, left, node.val) and 
                valid(node.right, node.val, right))
                
    return valid(root, float("-inf"), float("inf"))