【LeetCode】Letter Combinations of a Phone Number

Letter Combinations of a Phone Number 
Total Accepted: 4387 Total Submissions: 17627 My Submissions
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.
【解题思路】
        还是最基本的DFS，但是有一点需要注意，输入有可能为""，这个比较变态，感觉这个不应该是考点之一。
        将数字和字符的对应关系组成了一个数组
        String array[] = {"","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};
        这样处理起来相对要方便些。特别要注意digits中为数字的再转换，否则的话继续。
Java AC

public class Solution {
    public String array[] = {"","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};
    public ArrayList<String> letterCombinations(String digits) {
        ArrayList<String> list = new ArrayList<String>();
        if(digits == null){
            return list;
        }
        char numArr[] = digits.toCharArray();
        int len = digits.length();
        StringBuffer sb = new StringBuffer();
        dfs(list,sb,0,numArr,len);
        return list;
    }
    public void dfs(ArrayList<String> list,StringBuffer sb,
                int tempLen,char numArr[],int len){
        if(tempLen == len){
            list.add(sb.toString());
            return;
        }
        if(numArr[tempLen] >= '0' && numArr[tempLen] <= '9' ){
            String tempStr = array[numArr[tempLen] - '0'];
            int strLen = tempStr.length();
            for(int i = 0; i < strLen; i++){
                StringBuffer newsb = new StringBuffer(sb);
                newsb.append(String.valueOf(tempStr.charAt(i)));
                dfs(list,newsb,tempLen+1,numArr,len);
            }
        }else{
            StringBuffer newsb = new StringBuffer(sb);
            newsb.append(String.valueOf(numArr[tempLen]));
            dfs(list,newsb,tempLen+1,numArr,len);   
        }
    }
}