249. Group Shifted Strings

https://www.geeksforgeeks.org/group-shifted-string/


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) {
        // build a hashmap , key is a string of char differene betweens nei chars 
        // value is a list of string with the same key 
        
        HashMap<String, List<String>> map = new HashMap();
        for(String word : strings){
            String key = helper(word);
            if(!map.containsKey(key)){
                map.put(key, new ArrayList<>());
            }
            map.get(key).add(word);
        }
        List<List<String>> res = new ArrayList<>();
        for(String key : map.keySet()){ // keySet, not KeySet
            List<String> list = map.get(key);
            Collections.sort(list);
            res.add(list);
        }
        return res;
    }
    private String helper(String word){
        StringBuilder sb = new StringBuilder();
        for(int i = 0; i < word.length() - 1; i++){
            int first = word.charAt(i) - 'a';
            int sec = word.charAt(i + 1) - 'a';
            int diff = first - sec;
            if(diff < 0) diff += 26;
            sb.append(diff);
        }
        return sb.toString();
    }
}