list组合的问题，求源码

Question

String [] str = {"a","b","c","d"};
求这四个元素能组成的不重复list   a，b和 b，a   算同一个，求源码

Answer 1

import java.util.HashSet;
import java.util.Set;

public class ABCTest {
    public static void main(String[] args) {
        String[] str = {"a","b","c","d"};
        Set<String> list = new HashSet<>();
        for (int i = 0; i < str.length; i++) {
            list.add(str[i]);
            for (int j = i+1; j < str.length; j++) {
                list.add(str[i]+str[j]);
                for (int k = j+1; k < str.length; k++) {
                    list.add(str[i]+str[j]+str[k]);
                    for (int l = k+1; l < str.length; l++) {
                        list.add(str[i]+str[j]+str[k]+str[l]);
                    }
                }

            }
        }
        System.out.println(list);
    }
}

输出结果：[a, ab, bc, cd, abd, ac, b, bcd, bd, abc, acd, ad, c, d, abcd]

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