check BST python

# http://code2begin.blogspot.com
# Program to check if a given binary tree is a binary search tree

# node class
class node:
    def __init__(self, element):
        self.data = element
        self.left = None
        self.right = None


# function to check if the given binary tree is a BST or not
def check_BST(current):
    if current is None:
        return True

    if current.left != None and current.data < current.left.data :
        return False

    if current.right != None and current.data > current.right.data :
        return False

    return (check_BST(current.left) and check_BST(current.right));


head = node(1)
head.left = node(2)
head.right = node(3)
head.left.left = node(4)
head.left.right = node(5)
head.right.right = node(6)
head.left.left.right = node(7)
head.right.right.left = node(8)
head.left.left.right.left = node(9)
head.left.left.right.left.left = node(10)
head.right.right.left.right = node(11)

head2 = node(5)
head2.left = node(2)
head2.right = node(12)
head2.left.left = node(-4)
head2.left.right = node(3)
head2.right.left = node(9)
head2.right.right = node(21)
head2.right.right.left = node(19)
head2.right.right.right = node(25)

head3 = node(1)
head3.left = node(2)
head3.right = node(3)
head3.left.left = node(4)
head3.left.right = node(5)
head3.right.right = node(6)
head3.left.left.right = node(7)
head3.right.right.left = node(8)
head3.left.left.right.left = node(9)
head3.left.left.right.left.left = node(10)
head3.right.right.left.right = node(11)

print("Binary Tree #1 is a Binary Search Tree : " + str(check_BST(head)))
print("Binary Tree #2 is a Binary Search Tree : " + str(check_BST(head2)));
print("Binary Tree #3 is a Binary Search Tree : " + str(check_BST(head3)));