Cisco Coding Interview: Look-and-Say Next Term

You are asked to implement the next term of the look-and-say (aka count-and-say) sequence for a given digit string. Given a non-empty input string N containing only characters '0'–'9', you must scan N left-to-right, group contiguous identical digits, and for each maximal group append the group's decimal count followed by the digit. For example, input "1211" becomes "111221" because you read one '1', one '2', then two '1's.

Start by clarifying constraints: digits only, non-empty, counts are written in base-10 and concatenated directly. A typical interview flow is: (1) explain approach and edge cases (single char, long runs, very long input), (2) write a correct two-pointer or single-pass solution (maintain current digit and count), (3) discuss complexity and memory trade-offs, and (4) run through tests and corner cases.

Skills this question surfaces: string processing and run-length encoding, two-pointers or single-pass scanning, careful handling of integer-to-string conversion for counts, and time/space complexity reasoning (O(n) time, O(n) output). You should also show defensive thinking about large counts, streaming input, and how to extend the function to produce the k-th term of the sequence. Implementations in interviews are expected to be clear, correct, and accompanied by test cases.