java - 创建递归方法的返回类型

Question

我有以下递归方法，可以为给定字符串生成排列。我正在尝试为数组列表中生成的字符串创建返回类型，更具体地说，我正在尝试打印 jsp 页面中的输出。

 public static void permutation(String str) { 
      permutation("", str); 
 }

private static void permutation(String prefix, String str) {
     int n = str.length();
     if (n == 0) System.out.println(prefix);
     else {
          for (int i = 0; i < n; i++)
        permutation(prefix + str.charAt(i), str.substring(0, i) + str.substring(i+ n));
        }
}

Answer 1

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

public class Permute {

    public static void main(String[] args) {
        System.out.println(findAllPermutations("abc"));
    }

    public static List<String> findAllPermutations(String s) {
        if (s == null) {
            throw new NullPointerException();
        }
        if (s.length() <= 1) {
            return Arrays.asList(s);
        }

        List<String> permutations = new ArrayList<>();
        for (String permutation : findAllPermutations(s.substring(1))) {
            char ch = s.charAt(0);
            for (int i = 0; i <= permutation.length(); i++) {
                String prefix = permutation.substring(0, i);
                String suffix = permutation.substring(i);
                permutations.add(prefix + ch + suffix);
            }
        }
        return permutations;
    }
}