public class Solution { public String reverse(String input) { int len = input.length(); if (len == 0) { return ""; } char[] array = input.toCharArray(); swapHelper(array, 0, len - 1); return new String(array); } private void swapHelper(char[] array, int left, int right) { if (left >= right) { return; } swap(array, left, right); swapHelper(array, left + 1, right - 1); } private void swap(char[] array, int i, int j) { char temp = array[i]; array[i] = array[j]; array[j] = temp; } }