字符串的排列

Java的实现配合上牛客网无解的题目模板（返回值ArrayList<String>）

import java.util.*;
import java.util.stream.*;

public class Solution {
    public ArrayList<String> Permutation(String str) {
        ArrayList<String> ret = new ArrayList<>();
        if (str == null || str.length() == 0) {
            return ret;
        }
        perm(0, str.toCharArray(), ret);
        List<String> res = ret.stream().distinct().sorted().collect(Collectors.toList());
        return new ArrayList<>(res);
    }

    private void perm(int pos, char[] a, ArrayList<String> ret) {
        if (pos + 1 == a.length) {
            ret.add(new String(a));
            return;
        }
        for (int i = pos; i < a.length; ++i) {
            char tmp = a[i];
            a[i] = a[pos];
            a[pos] = tmp;

            perm(pos + 1, a, ret);

            tmp = a[i];
            a[i] = a[pos];
            a[pos] = tmp;
        }
    }
}