Given a string containing uppercase characters A-Z, remove pairs of adjacent characters if they are consecutive characters in the alphabet (for example, "AB" or "BA") until no more characters can be removed.
Return the resultant string.
The problem asks you to repeatedly remove any adjacent pair of uppercase letters that are consecutive in the alphabet, such as "AB" or "BA", until the string becomes stable. A standard solution is to scan the string with a stack: compare each character with the stack top, and pop when the two letters differ by exactly 1 in ASCII; otherwise push the character. This handles cascading removals efficiently in linear time.
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