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.
题解：
这道题也是来算一种combination的题，跟 Combinations 和 Word Break II 都比较类似（其实和leetcode里面很多题都相似）。
代码如下：
1 public ArrayList<String> letterCombinations(String digits) {
2 ArrayList<String> result=new ArrayList<String>();
3 if (digits==null)
4 return result;
5
6 String[] keyboard={"","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};
7 StringBuilder current=new StringBuilder();
8
9 int index=0;
buildResult(digits, index, current, keyboard, result);
return result;
}
private void buildResult(String digits, int index, StringBuilder current, String[] keyboard, ArrayList<String> result){
if (index==digits.length()){
result.add(current.toString());
return;
}
int num=digits.charAt(index)-'0';//get integer number
for (int i=0; i<keyboard[num].length(); i++){
current.append(keyboard[num].charAt(i));
buildResult(digits, index+1, current, keyboard, result);
current.deleteCharAt(current.length()-1);
}
}
Refrence：http://rleetcode.blogspot.com/2014/02/letter-combinations-of-phone-number-java.html