code: public StrBuilder reverse() {
          if (size == 0) {
              return this;
          }
  
          final int half = size / 2;
          final char[] buf = buffer;
          for (int leftIdx = 0, rightIdx = size - 1; leftIdx < half; leftIdx++,rightIdx--) {
              final char swap = buf[leftIdx];
              buf[leftIdx] = buf[rightIdx];
              buf[rightIdx] = swap;
          }
          return this;
      }
docs: Reverses the string builder placing each character in the opposite index.
  
  @return this, to enable chaining