String Reversed

Given a string S, return the "reversed" string where all characters that are not a letter stay in the same place, 
and all letters reverse their positions.

Example 1:

Input: "ab-cd"
Output: "dc-ba"
Example 2:

Input: "a-bC-dEf-ghIj"
Output: "j-Ih-gfE-dCba"
Example 3:

Input: "Test1ng-Leet=code-Q!"
Output: "Qedo1ct-eeLg=ntse-T!"
 
Note:

S.length <= 100
33 <= S[i].ASCIIcode <= 122 
S doesn't contain \ or "

func reverseOnlyLetters(_ S: String) -> String {
        
    }