Leetcode 17. Letter Combinations of a Phone Number, Java

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//'main' method must be in a class 'Rextester'.
//Compiler version 1.8.0_111

import java.util.*;
import java.lang.*;

//Letter Combinations of a Phone Number
class Rextester
{  
    Map<String,String> phone = new HashMap<String,String>();

List<String> out = new ArrayList<>();

public static void main(String args[])
    {
        System.out.println("Hello, World!");
    }
    public List<String> letterCombinations(String digits) {
        phone.put("2","abc");
            phone.put("3","def");
            phone.put("4","ghi");
            phone.put("5","jkl");
            phone.put("6","mno");
            phone.put("7","pqrs");
            phone.put("8","tuv");
            phone.put("9","wxyz");
        if(digits.length()!=0)
            helper("",digits);
        return out;
    }

public void helper(String combination, String digits){
            if(digits.length()==0){
                out.add(combination);
                return;
            }
            for(char c:phone.get(digits.substring(0,1)).toCharArray()){
                helper(combination+c,digits.substring(1));
            }
    }

}

[ + ] Show input
Compilation time: 0.73 sec, absolute running time: 0.15 sec, cpu time: 0.13 sec, memory peak: 18 Mb, absolute service time: 0,89 sec