Given a list of sorted characters consisting of both Uppercase and Lowercase Alphabets and a particular target value, say K, the task is to find the smallest element in the list that is larger than K. 
Letters also wrap around. For example, if K = ‘z’ and letters = [‘A’, ‘r’, ‘z’], then the answer would be ‘A’.

Input : Letters = ["D", "J", "K"]
       K = "B"
Output: 'D'
Explanation:
The Next greater character of "B" is 'D'
since it is the smallest element from the 
set of given letters, greater than "B".

smallest alphabet greater than a given character  : https://www.geeksforgeeks.org/smallest-alphabet-greater-than-a-given-character/