【leetcode刷题笔记】Letter Combinations of a Phone Number

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：

 public class Solution {
     public List<String> letterCombinations(String digits) {
         ArrayList<String> answer = new ArrayList<String>();

         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'}};
         String currentPath = new String();

         DeepSearch(map, answer, digits, currentPath);
         return answer;
     }

     public void DeepSearch(char[][] map,List<String> answer,String digits,String currentPath){
         if(digits.length() == 0)
         {
             String temp = new String(currentPath);
             answer.add(temp);

             return;
         }

         int now = digits.charAt(0) - '0';
         for(int j = 0;j < map[now-2].length;j++){
             currentPath += map[now-2][j];
             DeepSearch(map, answer, digits.substring(1), currentPath);
             currentPath = currentPath.substring(0, currentPath.length()-1);
         }
     }
 }