249. Group Shifted String

题目：
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"],
A solution is:

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

解答：

public List<List<String>> groupStrings(String[] strings) {
        List<List<String>> result = new ArrayList<List<String>>();
        Map<String, List<String>> map = new HashMap<String, List<String>>();
        
        for (String str : strings) {
            String key = "";
            for (int i = 1; i < str.length(); i++) {
                int offset = str.charAt(i) - str.charAt(i - 1);
                //在为负数的时候，当经过z, a的时候，数值大小会很大得反转
                key += offset < 0 ? offset + 26 : offset;
            }
            if (!map.containsKey(key)) map.put(key, new ArrayList<String>());
            map.get(key).add(str);
        }
        
        for (List<String> list : map.values()) {
            result.add(list);
        }
        return result;
    }