flutter 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!”
 *
 */
 
 String reverseOnlyLetters(String s) {
 }
 
 void main() {
  final result = reverseOnlyLetters("a-bC-dEf-ghIj");
   print(result);
   
}