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!"

    public String reverseOnlyLetters(String S) {
        StringBuffer sb = new StringBuffer(S);
        String letterS = "";
        for (int i = S.length() - 1; i >= 0; i--) {
            char ch = S.charAt(i);
            if (Character.isLetter(S.charAt(i))) {
                letterS += ch;
            }
        }
        int p = 0;
        for (int i = 0; i < S.length(); i++) {
            if (Character.isLetter(S.charAt(i))) {
                sb.setCharAt(i, letterS.charAt(p++));
            }
        }
        return sb.toString();
    }