Method helper

Gfg/Java/reverse-string.java:2–8

(char[] s, int left, int right)

class Solution {
  public void helper(char[] s, int left, int right) {
    if (left >= right) return;
    char tmp = s[left];
    s[left++] = s[right];
    s[right--] = tmp;
    helper(s, left, right);
  }

  public void reverseString(char[] s) {
    helper(s, 0, s.length - 1);