Devu and decorating birthday cake

Today is Devu's birthday. He has obtained few colored balloons from his friends. You are given this information by a string s consisting of lower case English Latin letters. Each letter (from 'a' to 'z') denotes a color. e.g. if s = "aab", then it means that he has received two balloons of color 'a' whereas one balloon of color 'b'. Now, Devu wants to decorate the cake by arranging all the balloo