Leetcode 17 Letter Combinations of a Phone Number - DFS思想

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"].

解题思路: 

依旧是DFS模型~注意把数字 0 - 9存进一个string array里面~使用stringbuilder比直接用string方便得多啊。。for循环里面是对array里面的某一个字符串的处理~

直接上代码:

 1 public ArrayList<String> letterCombinations(String digits) {
 2         String[] map = {"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
 3         ArrayList<String> ret = new ArrayList<String>();
 4         StringBuilder str = new StringBuilder();
 5         dfs(digits, map, 0, str, ret);
 6         return ret;
 7     }
 8     public void dfs(String digits, String[] map, int n, StringBuilder str, ArrayList<String> list){
 9         if(n == digits.length()){
10             list.add(str.toString());
11             return;
12         }
13         String s = map[digits.charAt(n) - ‘0‘];
14         for(int i = 0; i < s.length(); i++){    //注意i是小于s.lenght的
15             str.append(s.charAt(i));
16             dfs(digits, map, n + 1, str, list);
17             str.deleteCharAt(str.length() - 1);
18         }
19     }