Gargi and Strings

Gargi is thinking of a solution to a problem. Meanwhile, her friend asks her to solve another problem. Since Gargi is busy in her own problem, she seeks your help to solve the new problem. You are given a string S containing characters a-z (lower case letters) only. You need to change the string to a new string consisting of only one letter from a-z. For a given character S[i] in the string, if