Leetcode17 电话号码的字母组合

输入："23"
输出：["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].

class Solution {
    public List<String> letterCombinations(String digits) {
        List<String> res = new ArrayList<String>();
        if(digits == null || digits.length() == 0)
            return res;
        HashMap<Character,String> hashMap = new HashMap<>();
        hashMap.put('2',"abc");
        hashMap.put('3',"def");
        hashMap.put('4',"ghi");
        hashMap.put('5',"jkl");
        hashMap.put('6',"mno");
        hashMap.put('7',"pqrs");
        hashMap.put('8',"tuv");
        hashMap.put('9',"wxyz");
        trackback(res,hashMap,digits,0,new StringBuffer());
        return res;
    }
public void trackback(List<String> res,HashMap<Character,String> map,String digits,int index,StringBuffer s){
        if(index == digits.length()){
            res.add(s.toString());
        }else{
            String value = map.get(digits.charAt(index));
            for(int i = 0;i<value.length();i++){
                s.append(value.charAt(i));
                trackback(res,map,digits,index+1,s);
                s.deleteCharAt(index);
            }
        }

    }
}