打印字符串中所有字符的排列

输入一个字符串,按字典序打印出该字符串中字符的所有排列

public  static List<String> permutation4(String input){
        List<String> result = new ArrayList<>();
        if (input == null){
            return result;
        }
        if(input.length() == 1){
            result.add(input);
            return result;
        }

        for(int i = 0;i<input.length();i++){
            //获取第一个元素
            char first= input.charAt(i);
            System.out.println("first:"+first);
            //获取除第一个元素外的其他元素
            String end  = input.substring(0,i)+input.substring(i+1);
            System.out.println("end:"+end);
            //迭代，获取不同的排列组合
            List<String> restPerms =  permutation4(end);
            for(String param :restPerms){
                System.out.println("param:"+param);
                result.add(first+param);
            }
        }
        return result;
    }

first:a
end:bc
first:b
end:c
param:c
first:c
end:b
param:b
param:bc
param:cb
first:b
end:ac
first:a
end:c
param:c
first:c
end:a
param:a
param:ac
param:ca
first:c
end:ab
first:a
end:b
param:b
first:b
end:a
param:a
param:ab
param:ba
[abc, acb, bac, bca, cab, cba]