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.

Example:

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

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

class Solution {
    public List<List<String>> groupStrings(String[] strings) {
        List<List<String>> ans = new ArrayList<>();
        if (strings == null || strings.length == 0) return ans;
        Map<String, List<String>> hash = new HashMap<>();

        for (String str : strings){

            String key = getKey(str);

            if (!hash.containsKey(key)){
                hash.put(key, new ArrayList<String>());
            }

            hash.get(key).add(str);
        }

        return new ArrayList<>(hash.values());
    }

    private String getKey(String str){
        String key = "";
        for (int i = 1; i < str.length(); i++){
            int diff = str.charAt(i) - str.charAt(i - 1);
            if (diff < 0){
                diff += 26;
            }
            key += diff;
        }

        //System.out.println("str: " + str + " key: " + key);
        return key;
    }
}