DFS深度遍历 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"]

public class Solution {
public List<String> letterCombinations(String digits) {
Map<Character,String> map = new HashMap<Character,String>(){{
put('2', "abc"); put('3', "def"); put('4', "ghi"); put('5', "jkl");
put('6', "mno"); put('7', "pqrs"); put('8', "tuv"); put('9', "wxyz");
}};
List<String> res = new ArrayList<String>();
if(digits == null || digits.equals("")) {
return res;
}
dfs(digits, 0, "", map, res);
return res;
}

public void dfs(String digits, int index, String path, Map<Character,String> map, List<String> res) {
if(path.length() == digits.length()) {
res.add(path);
}
for(int i = index; i < digits.length(); i++) {
for(char c : map.get(digits.charAt(i)).toCharArray()) {
dfs(digits, i + 1, path + c, map, res);//遍历到最深一层，依次往上一层返回遍历
}
}
}
}