Mubashir was reading about Doubleton Numbers on Wikipedia.
A natural number is a "Doubleton Number", if it contains exactly two distinct digits. For example, 23, 35, 100, 12121 are doubleton numbers, and 123 and 114455 are not.
Create a function which takes a number n and finds the next doubleton number.
doubleton(10) ➞ 12
# 12 has only two distinct numbers 1 and 2
doubleton(120) ➞ 121
# 121 has only two distinct numbers 1 and 2
doubleton(1234) ➞ 1311
# 1311 has only two distinct numbers 1 and 3
N/A