題目原型：
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.
基本思路：
遞歸求解
public ArrayList letterCombinations(String digits)
{
ArrayList list = new ArrayList();
char[] number = digits.toCharArray();//存放電話號碼
int len = number.length;//電話號碼長度
String[] phone = {"","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};//字母與數字對應表
int[] total = {0,0,3,3,3,3,3,4,3,4};//每個數字上有多少個字母
int[] answer = new int[len];//指示每個數字所對應字母序列的字符位子,如第一個數字是2，如果answer[0] = 0,表示所對應的字符是a即phone[0].charAt[answer[0]]='a'
combinatons(list,phone, number, answer, 0, len, total);
return list;
}
void combinatons(ArrayList list,String[] phone,char[] number , int[] answer , int index , int len , int[] total)
{
if(index==len)
{
StringBuffer strbuf = new StringBuffer();
for(int i = 0 ;i
