'''
Implement a function to check if a binary tree is a binary search tree.

Hints: 35, 57, 86, 113, 128
'''


def validate_BST(s):
    pass

if __name__ == '__main__':
    assert validate_BST('') ==
    assert validate_BST('') ==
    assert validate_BST('') ==
    assert validate_BST('') ==
    assert validate_BST('') ==
