题目：

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.

题意：

给定一个数字字符串，返回这个数字能代表的所有的字母的组合。

数字对字母的映射如下图所示（就在电话的按键中）

Input:Digit string "23"
Output: ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].

返回的结果可以按照任意的顺序。

算法分析：

 * 思路:


 *比如“234”这个字符串，我可以先将0...1的所有排列找到-->{"a", "b", "c"}


 *再进一步将0...2的所有排列找到-->{"ad", "ae","af", "bd", "be", "bf", "cd", "ce", "cf"}


 *如此循环...直到字符串末尾。实现如下

AC代码：

public class Solution 
{
    public  ArrayList<String> letterCombinations(String digits) 
    {
    	ArrayList<String> res=new ArrayList<String>();
    	String charmap[] = {"0", "1", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
    	if (digits.length()==0) return res;
    	res.add("");
        for (int i = 0; i < digits.length(); i++)
        {
          ArrayList<String> tempres=new ArrayList<String>(); 
          String chars = charmap[digits.charAt(i) - '0'];
          for (int c = 0; c < chars.length();c++)
          {
            for (int j = 0; j < res.size();j++)
              tempres.add(res.get(j)+chars.charAt(c));
          }
            res = tempres;
        }
        return res;
    }
}