Letter Combinations of a Phone Number || 电话号码对应的单词

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.

public class Solution {
    public String []c={"","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};
    public int []total={0,0,3,3,3,3,3,4,3,4};
    public List<String> letterCombinations(String digits) {
        List<String> res=new LinkedList<String>();
        dfs(digits,"",0,res);
        return res;
    }
    public void dfs(String digits,String ans,int index,List<String> res){
        if(index==digits.length()){
            res.add(ans);
            return;
        }
        int i=digits.charAt(index)-'0';
        if(i<2 || i>10) return;
        for(int j=0;j<total[i];j++){
            dfs(digits,ans+String.valueOf(c[i].charAt(j)),index+1,res);
        }
    }
}