Stingy Strings

Problem Statement It is 2042 and we are surrounded by cyborgs everywhere. Geneo is one such cyborg with a human body and an electronic brain. To monitor skills and health of Geneo he is posed with questions everyday. On one fine day Geneo was asked the following question. You are given a string S of size N containing letters a to z. You are allowed to remove a character from the string and excha