Two Problem Coding Screen

What they ask

There is no local engineering office, so the whole loop runs remotely with a recruiter and interviewers in another region. After the recruiter call comes the technical screen: 45 minutes in CoderPad with the run button switched off, about 35 minutes of actual coding, and two problems is the norm. The interviewer expects a working solution to both, so the pace is fast.

The problems come from the well known company tagged lists. One from a 2025 loop (the screen and the onsite coding rounds draw from the same pool), rewritten:

Given a string s of letters and digits, return every string you can make by independently choosing upper or lower case for each letter. Digits stay as they are. Order does not matter.

The second slot in that loop was a tree problem (see the subtree average question), and other candidates report array and hashing questions, interval merging and k closest points.

Examples

• "a1b" returns ["a1b", "A1b", "a1B", "A1B"].
• "123" returns ["123"].

Constraints

• 1 <= len(s) <= 12, so the output can have up to 4096 strings.

What they look for

• Clarifying the question in under a minute, then stating the approach before writing it.
• A clean backtracking (or iterative doubling) solution with O(2^L * n) time stated, where L is the number of letters.
• Verification by hand: because nothing runs, you pick a small input, walk through your code line by line, and catch your own bugs. A candidate who finishes both problems but never traces them is usually a no.
• Four signals are graded: coding, problem solving, communication and verification. Speed matters because the second problem is where most of the signal comes from.

Dynamic programming is rarely asked at the screen; arrays, strings, hashing, trees and graphs are the usual territory.