give string s consist lowercase English letter return letter appear twiceNoteA letter appear twice letter b second occurrence second occurrence bs contain letter appear twice Example 1input s abccbaaczoutput cexplanationthe letter appear index 0 5 6the letter b appear index 1 4the letter c appear index 2 3 7the letter z appear index 8the letter c letter appear twice letter index second occurrence smallestexample 2Input s abcddoutput dexplanationthe letter appear twice d return d constraints2 slength 100s consist lowercase English letterss repeat letter