344. 反转字符串

    //迭代
    public void reverseString(char[] s) {
        if (s == null || s.length < 2) {
            return;
        }
        int left = -1;
        int right = s.length;
        char temp;
        while (++left < --right) {
            temp = s[left];
            s[left] = s[right];
            s[right] = temp;
        }
    }

    // 尾递归
    public void reverseString2(char[] s) {
        helper(s, 0, s.length - 1);
    }

    public void helper(char[] s, int start, int end) {
        if (start >= end) {
            return;
        }

        char temp = s[start];
        s[start] = s[end];
        s[end] = temp;

        helper(s, start + 1, end - 1);
    }