Leetcode #249. Group Shifted Strings

[yokostan]

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

        for (int i = 0; i < strings.length; i++) {
            String str = shift(strings[i]);
            if (!map.containsKey(str)) map.put(str, new ArrayList<Integer>());
            map.get(str).add(i);
        }

        for (String str : map.keySet()) {
            ArrayList<String> list = new ArrayList<>();
            for (int i = 0; i < map.get(str).size(); i++) {
                list.add(strings[map.get(str).get(i)]);
            }
            res.add(list);
        }

        return res;
    }

    private String shift(String str) {
        StringBuilder sb = new StringBuilder();
        int gap = str.charAt(0) - 'a';
        for (int i = 0; i < str.length(); i++) {
            int c = (int)str.charAt(i) - gap;
            if ((char)c < 'a') c += 26;
            sb.append((char)c);
        }

        return sb.toString();
    }
}

Storing ArrayList as value in the map will make things easier! Also that even saves us some space. String bytesin Java: Internally - it contains (number of chars) * 2 bytes, as each char in Java takes up two bytes (a normal character in Java is 16 bits unicode). The actual bytes are 0x20 and 0x14. While int is 4 bytes.