class Solution {
    public String reverseString(String s) {
        if (s == null || s.length() == 0) {
            return s;
        }
        char[] array = s.toCharArray();
        for (int i = 0; i < array.length / 2; i++) {
            swap(array, i, array.length - 1 - i);
        }
        return new String(array);
    }
    private void swap(char[] array, int i, int j) {
        char tmp = array[i];
        array[i] = array[j];
        array[j] = tmp;
    }
}