public class ReverseString {
    public String reverse(String s) {
        char[] arr = s.toCharArray();
        int l = 0, r = arr.length - 1;
        while (l < r) {
            char tmp = arr[l];
            arr[l++] = arr[r];
            arr[r--] = tmp;
        }
        return String.valueOf(arr);
    }
}

public String reverse(String s) {
    return new StringBuilder(s).reverse().toString();
    return new StringBuffer(s).reverse().toString();
}