give stre english letter s return great english letter occur lowercase uppercase letter s the return letter uppercase if letter exist return stringan english letter b great letter b appear english alphabet Example 1input s leetcodeoutput eexplanationthe letter e letter appear low upper caseExample 2Input s arRAzFifOutput rexplanationthe letter r great letter appear low upper casenote a F appear low upper case r great F AExample 3input s abcdefghijkoutput ExplanationThere letter appear low upper case Constraints1 slength 1000 consist lowercase uppercase english letter