917. Reverse Only Letters

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:

1. S.length <= 100

2. 33 <= S[i].ASCIIcode <= 122

3. S doesn't contain \ or "

class Solution {
    public String reverseOnlyLetters(String S) {
        if(S == null || S.isEmpty()) return S;
        
        char[] charArr = S.toCharArray();
        
        
        Stack<Character> stack = new Stack();
       for(char c : charArr){
           if(Character.isLetter(c))
               stack.push(c);
       }
        
        StringBuilder sb = new StringBuilder();
        
        for(char c : charArr){
            if(Character.isLetter(c)) sb.append(stack.pop());
            else sb.append(c);
        }
        
        return sb.toString();
        
        
     }
}