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.

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题解：用一个二维数组map保存数字到字母的映射，然后深度优先递归搜索如下的一棵树（以"23"为例，树不完整）

代码如下：

1 public class Solution { 2     public List
     
       letterCombinations(String digits) { 3         ArrayList
      
        answer = new ArrayList
       
        (); 4          5         char[][] map = {
   {'a','b','c'},{'d','e','f'},{'g','h','i'},{'j','k','l'},{'m','n','o'},{'p','q','r','s'},{'t','u','v'},{'w','x','y','z'}}; 6         String currentPath = new String(); 7          8         DeepSearch(map, answer, digits, currentPath); 9         return answer;10     }11     12     public void DeepSearch(char[][] map,List
        
          answer,String digits,String currentPath){13         if(digits.length() == 0)14         {15             String temp = new String(currentPath);16             answer.add(temp);17 18             return;19         }20         21         int now = digits.charAt(0) - '0';22         for(int j = 0;j < map[now-2].length;j++){23             currentPath += map[now-2][j];24             DeepSearch(map, answer, digits.substring(1), currentPath);25             currentPath = currentPath.substring(0, currentPath.length()-1);26         }27     }28 }