[LeetCode] 17. 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"].

Note:
Although the above answer is in lexicographical order, your answer could be in any order you want.

public class Solution {
    
    public void letterCombinationsAux(String digits, int i, char[][] dial, StringBuilder sb, List res){
        
        int len = digits.length();
        
        if(i >= len){
            res.add(sb.toString());
            return;
        }
        
        int number = Character.getNumericValue(digits.charAt(i));
        
        for(char c : dial[number]){
            sb.append(c);
            letterCombinationsAux(digits, i+1, dial, sb, res);
            sb.deleteCharAt(sb.length()-1);
        }
        
        return;
        
    }
    
    public List letterCombinations(String digits) {
        char[][] dial = {{}, {}, {'a','b','c'},{'d','e','f'}, {'g','h','i'},{'j','k','l'},{'m','n','o'},{'p','q','r','s'},{'t','u','v'},{'w','x','y','z'}};
        
        int len = digits.length();
        ArrayList res = new ArrayList();
        
        if(0 == len)
            return res;
            
        letterCombinationsAux(digits, 0, dial, new StringBuilder(), res);
        
        return res;
    }
}

public class Solution {
    
    public void letterCombinationsAux(String digits, int i, char[][] dial, char[] path, List res){
        
        int len = digits.length();
        
        if(i >= len){
            res.add(new String(path));
            return;
        }
        
        int number = Character.getNumericValue(digits.charAt(i));
        
        for(char c : dial[number]){
            path[i] = c;
            letterCombinationsAux(digits, i+1, dial, path, res);
        }
        
        return;
        
    }
    
    public List letterCombinations(String digits) {
        char[][] dial = {{}, {}, {'a','b','c'},{'d','e','f'}, {'g','h','i'},{'j','k','l'},{'m','n','o'},{'p','q','r','s'},{'t','u','v'},{'w','x','y','z'}};
        
        int len = digits.length();
        List res = new ArrayList();
        
        if(0 == len)
            return res;
            
        letterCombinationsAux(digits, 0, dial, new char[len], res);
        
        return res;
    }
}