Transform String

Problem Overview

• Given a string S of A, B, C, and D, remove adjacent pairs A with B or C with D repeatedly until no more removals are possible, and return the resulting string.
• Input: S with length 0 to 250,000 using only A, B, C, D; Output: any valid irreducible string.
• Domain: string processing and reduction where specified adjacent patterns are eliminated.
• From Janestreet interviews, this coding interview problem is an interview question with multiple valid outcomes.