Below are the permutations of string ABC.
ABC ACB BAC BCA CBA CAB
idea: 交换位置，当index == last index of string时说明已经到了最后一位，可以打印
code：
private void helper(String s, int start) {
if (start == s.length() - 1) {
System.out.println(s);
} else {
for (int i = start; i < s.length(); i++) {
//核心三句话
s = swap(s, start, i);
helper(s, start + 1);
s = swap(s, start, i);
}
}
}