*Group Shifted Strings

Given a string, we can "shift" each of its letter to its successive letter, for example: "abc" -> "bcd". We can keep "shifting" which forms the sequence:

"abc" -> "bcd" -> ... -> "xyz"

Given a list of strings which contains only lowercase alphabets, group all strings that belong to the same shifting sequence.

For example, given: ["abc", "bcd", "acef", "xyz", "az", "ba", "a", "z"], 
Return:

[
  ["abc","bcd","xyz"],
  ["az","ba"],
  ["acef"],
  ["a","z"]
]

 public class Solution {
    public List<List<String>> groupStrings(String[] strings) {

        List<List<String>> res = new ArrayList<List<String>>();
        HashMap<String, List<String>> map = new HashMap<String, List<String>>();

        for(String word : strings){
            String key = "";
            int offset = word.charAt(0) - 'a';
            for(int i = 1; i < word.length(); i++){
                key += (word.charAt(i) - offset + 26) % 26;
            }

            if(!map.containsKey(key)){
                map.put(key, new ArrayList<String>());
            }
            map.get(key).add(word);
        }

        for(List<String> list : map.values()){
            Collections.sort(list);
            res.add(list);
        }

        return res;

    }
}

reference: https://leetcode.com/discuss/67240/around-13-lines-code-in-java