西小瓜: Leetcode: Letter Combinations of a Phone Number

Given a digit string, return all possible letter combinations that the number could represent.
A mapping of digit to letters (just like on the telephone buttons) is given below.

Input:Digit string "23"
Output: ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].
Although the above answer is in lexicographical order, your answer could be in any order you want.

    public ArrayList<String> letterCombinations(String digits) {

        ArrayList<String> results=new ArrayList<String>();

        StringBuilder re=new StringBuilder();

        String[] board=new String[10];

        board[0]=" ";board[1]="1";board[2]="abc";

        board[3]="def";board[4]="ghi";board[5]="jkl";

        board[6]="mno";board[7]="pqrs";board[8]="tuv";board[9]="wxyz";

        helper(results,re,digits,board,0);

        return results;

    }

    public void helper(ArrayList<String> results, StringBuilder re, String digits, String[] board, int step){

        if(step==digits.length()){

            results.add(re.toString());

            return;

        }

        String s=board[digits.charAt(step)-'0'];

        //or

        //int num=digits.charAt(step);

        //String s=board[num];

        for(int i=0;i<s.length();i++){

            re.append(s.charAt(i));

            helper(results,re,digits,board,step+1);

            re.deleteCharAt(step);

        }

    }

Read full article from 西小瓜: Leetcode: Letter Combinations of a Phone Number