Given a string composed of both lowercase and uppercase letters. Sort it in such a manner such that the uppercase and lowercase letter comes in an alternate manner but in sorted way.

Example 1:

Input:
S = bAwutndekWEdkd
Output: AbEdWddekkntuw
Explanation: letters A,E,W are sorted 
as well as letters b,d,d,d,e,k,k,n,t,u,w are 
sorted and both appears alternately in the 
string as far as possible.

​Example 2:

Input: 
S = AiBFR
Output: AiBFR
Explanation: letters A,B,F,R and sorted 
as well as letter i. 

User Task:
You don't need to read input or print anything. You just have to complete the function stringSort () which takes a string as input and returns the sorted string as described in the problem description.

Expected Time Complexity: O(|S|).
Expected Auxiliary Space: O(1). 

Constraints:
1<=|S|<=1000