Lexicographically Smallest String After Substring Operation

You are handed a string s that contains nothing but lowercase English letters. Pick any non-empty substring of s — a contiguous run of one or more characters — and then move every character inside that run one letter earlier in the alphabet, applying the change exactly once to each character: for instance, 'b' turns into 'a' and 'c' turns into 'b'. The alphabet is circular, so stepping 'a' backward produces 'z'. Characters located outside the chosen run are left completely…