电话号字母组合，利用深度搜索的思想。

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"].

import java.util.ArrayList;
import java.util.List;

public class test
{
          List<String> res;
        public List<String> letterCombinations(String digits) {
            String[] table = {"","", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
            char[] temp = new char[digits.length()];
            res = new ArrayList<String>();
            helper(table, 0, temp, digits);
            return res;
        }
        
        public void helper(String[] table, int index, char[] temp, String digits)
        {
            if(index == digits.length())
            {
                if(temp.length!=0)//判断，不然A不过去。
                {
                    res.add(new String(temp));
                }
            }
            else
            {
                String candidates = table[digits.charAt(index)-'0'];//候选字母表
                for(int i = 0; i < candidates.length(); i ++)
                {
                    temp[index] = candidates.charAt(i);
                    helper(table, index + 1, temp, digits);//深度遍历digits
                }
            }
        }
        
        public static void main(String[] args)
        {
            test t = new test();
            System.out.println(t.letterCombinations("23"));
        }
}