Let us consider the string in problem as S and the string that we generate as T.
What my solution does it:
Solve(1,S.size()):
Ans=0
1)If the first and last character of S and T are same,Ans+=Solve(2,s.size()-1)
2)If the first and last character of T are greater than S, then we can place any charcters from [2,S.size()-1].
3)If first character of T =first character of S,but last character of T is > last character of S,then we have to make sure that T[2,S.size()-2]>=S[2,S.size()-2]
4)If first character of T > first character of S,but last character of T is = last character of S,then we have to make sure that substring T[S.size()-2,2]>=S[S.size()-2,2]
Base Conditions: Solve(x,y) if x<y return 0
Solve(x,y) if x==y return ('Z'-S[x])
Here A[x,y] denotes substring of A from index x to y inclusive.
Tough to understand the idea. But code gives wrong output for "YYYY". Correct output will be 735. If you correct your solution from WA, then it will give TLE. So my advice is to see Editorial for more details. Editorial