I applied at a career fair and then received an email for an on-campus interview.

The process has been very quick, only two weeks so far.

They will notify candidates for on-site interviews within a few days after the on-campus interview.

Write a function to find how many valid letters are in a string of digits, given the mapping:

a -> 1 b -> 2 ... z -> 26

Example: '123' can be decoded as:

• 1, 2, 3 -> 'a', 'a', 'c'
• 12, 3 -> 'l', 'c'
• 1, 23 -> 'a', 'w'

This function should count the number of unique valid letter combinations.