hashhh

Map<Character, List<Integer>> freqMap1 = new HashMap<>();
        Map<Character, List<Integer>> freqMap2 = new HashMap<>();

        for (int i=0; i<s1.length(); i++) {
            if (freqMap1.containsKey(s1.charAt(i))) {
                freqMap1.get(s1.charAt(i)).add(i);
            } else {
                freqMap1.put(s1.charAt(i), new ArrayList<>(Arrays.asList(i)));
            }

        }

        for (int i=0; i<s2.length(); i++) {
            if (freqMap2.containsKey(s2.charAt(i))) {
                freqMap2.get(s2.charAt(i)).add(i);
            } else {
                freqMap2.put(s2.charAt(i), new ArrayList<>(Arrays.asList(i)));
            }
        }

        List<List<Integer>> freqList1 = new ArrayList<>();
        List<List<Integer>> freqList2 = new ArrayList<>();

        for (Map.Entry entry: freqMap1.entrySet()) {
            int index=0;
            freqList1.get(index).add(entry.getValue());
            index++;
        }