LeetCode-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"],
A solution is:

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

 

Solution:

public class Solution {
    public List<List<String>> groupStrings(String[] strings) {
        List<List<String>> resLists = new LinkedList<List<String>>();

        HashMap<String,List<String>> patternMap = new HashMap<String,List<String>>();
        for (String str : strings){
            StringBuilder builder = new StringBuilder().append(str);
            int delta = builder.charAt(0)-'a';
            builder.setCharAt(0,'a');
            for (int i=1;i<builder.length();i++){
                char c = (char) ((builder.charAt(i) + 26 - delta)%26);
                builder.setCharAt(i,c);
            }
            String pattern = builder.toString();
            if (!patternMap.containsKey(pattern)){
                patternMap.put(pattern,new LinkedList<String>());
            }
            patternMap.get(pattern).add(str);
        }

        for (List<String> strList : patternMap.values()){
            resLists.add(strList);
        }
        return resLists;
    }
}