Submit my resume at the campus career fair. Then I got an email to schedule a phone interview. Around the phone interview, I took an online test. The phone interview asks some common questions and about some projects I wrote on my resume. The online test has 4 parts:

• The first part is 10 math questions and should be finished in 10 minutes.
• The second part is some math questions.
• The third part asks you to learn a new programming language.
• The fourth part is 4 programming problems.

Produce all possible strings given a sequence of numbers. Each number represents 3 or 4 alphabetic characters, like the old mobile keyboard.

A company wants to give its employees phone numbers. There are several requirements:

1. Cannot have two same digits in a row.
2. If 4 is in the phone number, 4 has to be the 1st digit.

Given length n, print all possible numbers.