DFS做法， 能满足需求，有改进的地方欢迎大家提出

import java.util.*;
//Author: Zhenjie Hao

public class Main {
    public static void main(String[] args){

        int input = 123;
        List<List<String>> result = new ArrayList<>();

        result = converse(input);

        display(result);


        
    }

    public static List<List<String>> converse(int input){
        String str = String.valueOf(input);
        List<List<String>> result = new ArrayList<>();

        dfs(str, 0 , new ArrayList<>(), result);
        return result;
    }

    public static void dfs(String str, int startIndex, List<String> path, List<List<String>> result){

        if(startIndex == str.length()){
            result.add(new ArrayList<String>(path));
        }


        for(int i = startIndex + 1; i <= str.length(); i++){
            String sub = str.substring(startIndex, i);
            if(!isValid(sub)){
                break;
            }
            path.add(numToAlg(sub));
            dfs(str, i, path, result);
            path.remove(path.size()-1);
        }


    }


    public static boolean isValid(String str){
        //str = "12" or "27"
        if(str.charAt(0) == '0'){
            return false;
        }
        int num = Integer.parseInt(str);
        if(num < 1 || num > 26){
            return false;
        }
        return true;
    }

    public static String numToAlg(String str){
        //str = "12"
        //putput = "d"

        int num = Integer.parseInt(str);

        return String.valueOf((char)('a' + num - 1));

    }

    public static void display(List<List<String>> result){
        for(int i = 0; i < result.size(); i++){
            StringBuilder sb = new StringBuilder();
            for(int j = 0; j < result.get(i).size(); j++){
                sb.append(result.get(i).get(j));
                
            }
            System.out.println(sb.toString());
        }
    }

}