9.Letter Combinations of a Phone Number-电话号码的字母组合

题目描述：

输入输出描述：

解析：该题在LeetCode上是一个Medium题，像这种组合题，一般考虑回溯法，不过这次代码用的是哈希表

    class Solution {
        public List<String> letterCombinations(String digits) {
            //方法：哈希表
            HashMap<Character, String> hashMap = 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");
                }
            };
            ArrayList<String> res = new ArrayList<>();
            if(digits.length()==0) return res;
            res.add("");
            for (char c : digits.toCharArray()) {
                ArrayList<String> tmp = new ArrayList<>();
                String s = hashMap.get(c);
                for (char t : s.toCharArray()) {
                    for (String re : res) {
                        tmp.add(re+t);
                    }
                }
                res=tmp;
            }
            return res;
        }
    }