I received the phone interview directly after the referral, which took no more than two weeks.

The interviewer was nice and patient. When I got stuck, he kindly waited for me and gave me several hints. The process was also really fast; the day after the phone interview, I received the onsite interview invitation.

The question is quite easy. Check whether a given string has valid parentheses.

For example:

• { needs to match }
• ( matches )
• [ matches ]

And you have to check whether they are in the right order.

The follow-up question is to check the valid string again but with given pairs. Say the pairs are "ab cd". That means 'a' needs to match 'b' and 'c' needs to match 'd'.