Letter Combinations of a Phone Number leetcode java

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