Write a function that takes a string as input and returns the string reversed.

这个题也没啥说的，按道理直接StringBuilder然后reverse一下就可以了，既然是道题，估计可能需要转成一个char array一个一个交换

class Solution {
    public String reverseString(String s) {
        if(s == null || s.length() <= 1)
            return s;

        char[] cs = s.toCharArray();
        int i = 0;
        int j = cs.length - 1;

        while(i < j) {
            swap(cs, i++, j--);
        }

        return String.valueOf(cs);
    }

    public void swap(char[] cs, int i, int j) {
        char temp = cs[i];
        cs[i] = cs[j];
        cs[j] = temp;
    }
}