Leetcode 17 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"].


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:

public ArrayList<String> letterCombinations(String digits) {
        String[] map = {"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
        ArrayList<String> ret = new ArrayList<String>();
        StringBuilder str = new StringBuilder();
        dfs(digits, map, 0, str, ret);
        return ret;
    }
    public void dfs(String digits, String[] map, int n, StringBuilder str, ArrayList<String> list){
        if(n == digits.length()){
            list.add(str.toString());
            return;
        }
        String s = map[digits.charAt(n) - '0'];
        for(int i = 0; i < s.length(); i++){    //注意i是小于s.lenght的
            str.append(s.charAt(i));
            dfs(digits, map, n + 1, str, list);
            str.deleteCharAt(str.length() - 1);
        }
    }