human
Solve in Python:
Given a binary tree, determine if it is a valid binary search tree (BST).

Assume a BST is defined as follows:

       The left subtree of a node contains only nodes with keys less than the node's key.
       The right subtree of a node contains only nodes with keys greater than the node's key.
       Both the left and right subtrees must also be binary search trees.

Example 1:

Input:
    2
   / \
  1   3
Output: true

Example 2:

    5
   / \
  1   4
     / \
    3   6
Output: false
Explanation: The input is: [5,1,4,null,null,3,6]. The root node's value
             is 5 but its right child's value is 4.
 Can you provide a Python code to determine if it is a valid binary search tree (BST)?
assistant
#Here is Python code to determine if it is a valid binary search tree (BST).

# 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, node):
         """
         :type root: TreeNode
         :rtype: bool
         """
         def valid(somenode, ceiling, floor):
             if somenode is None:
                 return True
             if somenode.val >= ceiling or somenode.val <= floor:
                 return False
             
             
             return valid(somenode.left, somenode.val, floor) and valid(somenode.right, ceiling, somenode.val)
         
         return valid(node, sys.maxsize, -sys.maxsize)