Problem Statement 
 Given a String s, we will first collect all distinct positive length subsequences of s, and place them in a list L. A subsequence of s is obtained by deleting 0 or more characters. Next, sort L into ascending order by length. Where ties occur, break them lexicographically. Here uppercase letters occur before lowercase letters. Finally, return the String in position pos % k (0based), where k is the number of elements in L, and % is the modulus operator. 

Definition 
 Class:  GetSubsequence  Method:  getAt  Parameters:  String, String  Returns:  String  Method signature:  String getAt(String s, String pos)  (be sure your method is public) 




Constraints 
  s will contain between 2 and 50 characters inclusive. 
  Each character in s will be a letter ('A''Z','a''z'). 
  pos will be an integral value between 0 and 2^63  1 inclusive. 
  pos will not have extra leading zeros. 

Examples 
0)  
  Returns: "ABCD"  "ABCD" is the last of the 15 subsequences in L. 


1)  
  Returns: "ABD"  19203410239121 % 15 = 11. 


2)  
 
3)  
 "AbcdAbcdAbcd"  "39283423984923" 
 Returns: "AbAbdbd"  
