题目：输入字符串，输出该字符串的全排列。
样例：输入"abc",输出"abc,acb,bac,bca,cba,cab"

解法1：递归输出

    private static void stringPermutation(char[] str, int from, int to) {
        if (str == null || str.length == 0) {
            return;
        }
        if (from == to) {
            System.out.println(String.valueOf(str));
        } else {
            for (int i = from; i < to; i++) {
                swap(str, from, i);
                stringPermutation(str, from + 1, to);
                swap(str, from, i);
            }
        }
    }