lintcode-medium-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.

 

Given "23"

Return["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"]

 

public class Solution {
    /**
     * @param digits A digital string
     * @return all posible letter combinations
     */
    public ArrayList<String> letterCombinations(String digits) {
        // Write your code here
        if(digits == null || digits.length() == 0)
            return new ArrayList<String>();
        
        String[] strs = {" ", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
        
        ArrayList<String> result = new ArrayList<String>();
        StringBuilder line = new StringBuilder();
        
        helper(result, line, 0, digits, strs);
        
        return result;
    }
    
    public void helper(ArrayList<String> result, StringBuilder line, int start, String digits, String[] strs){
        if(start == digits.length()){
            result.add(line.toString());
            return;
        }
        
        int index = digits.charAt(start) - '0';
        for(int i = 0; i < strs[index].length(); i++){
            line.append(strs[index].charAt(i));
            helper(result, line, start + 1, digits, strs);
            line.deleteCharAt(start);
        }
        
        return;
    }
    
}