Valid BST

This problem was asked by LinkedIn.

Determine whether a tree is a valid binary search tree.

A binary search tree is a tree with two children, left and right, and satisfies the constraint that the key in the left child must be less than or equal to the root and the key in the right child must be greater than or equal to the root.

My Solution(Python):

from tree import Node
import random

class TreeNode(Node):
    def __init__(self, val=None):
        super().__init__(data=val)

def isValidBST(root):
    if root is None:
        return True
    if root.left is not None:
        c1 = root.left.data<=root.data
    else:
        c1 = True
    if root.right is not None:
        c2 = root.right.data>=root.data
    else:
        c2 = True
    return c1 and c2 and isValidBST(root.left) and isValidBST(root.right)


def main():
    tree = TreeNode(11); tree.left=TreeNode(10); tree.left.right=TreeNode(10.5); tree.right=TreeNode(12); tree.right.left=TreeNode(11.5)
    tree.prettyPrint()
    print('\n\n', isValidBST(tree))

if __name__=='__main__':
    main()