validate bst

is_valid_bst.py

#!/usr/bin/env python

class Node:

    def __init__(self, value):
        self.value = value
        self.right = None
        self.left = None

    def is_valid_bst(self, head=None):
        if self.left is None:
            return True
        if self.right is None:
            return True
        if self.left.value >= self.value:
            return False
        if self.right.value <= self.value:
            return False
        if head and (self.left.value + self.right.value) > head:
            return False
        return self.left.is_valid_bst(self.value) and self.right.is_valid_bst(self.value)

def main():
    a = Node(3)

    b = Node(2)
    a.left = b

    c = Node(5)
    a.right = c

    d = Node(1)
    b.left = d

    e = Node(4)
    b.right = e

    print(a.is_valid_bst())

if __name__ == '__main__':
    main()