code: public static int nthIndex(String s, char ch, int n) {
      int index = 0;
      for (int i = 0; i < n; i++) {
        // if we're already at the end of the string,
        // and we need to find another ch, return -1
        if (index == s.length() - 1) {
          return -1;
        }
        index = s.indexOf(ch, index + 1);
        if (index == -1) {
          return (-1);
        }
      }
      return index;
    }
docs: Returns the index of the <i>n</i>th occurrence of ch in s, or -1
  if there are less than n occurrences of ch.