Ela love read lot like new coworker DTL on day engineer DTL challenge coworker sort heap book different compartment shelfn book split k compartment bookshelf n divisible k each book represent lowercase Latin letter y inclusively begin letter title bookela stack exactly fracnk book compartment after book stack compartment index 1 k take minimum exclude MEX letter multiset letter form letter represent book compartment combine result letter stre the letter result stre MEX letter multiset letter form compartment second letter result stre MEX letter multiset letter form second compartment please note constraint problem MEX letter determine multiset find problem z usedwhat lexicographically great result string possible Ela createA string lexicographically great string b follow hold b prefix b ne position b differ string letter appear later alphabet corresponding letter b the minimum exclude MEX letter multiset letter letter appear early alphabet contain multiset for example multiset letter contain 7 letter b b c e c f respectively MEX letter compartment d d include multiset letter come d alphabet b c include multiset