public static String isValid(String s) {
        // Write your code here
        HashSet <Character> stringSet = new HashSet<>();
        List <Character> stringList = new ArrayList<>();
        HashMap <Character, Integer> frequency = new HashMap<>();
        HashMap <Integer, Integer> freqCount = new HashMap<>();
        for (char c : s.toCharArray()) {
            stringSet.add(c);
            stringList.add(c);
        }
        for (Character ch : stringSet) {
            frequency.put(ch, Collections.frequency(stringList, ch));
        }
        int size = new HashSet<>(frequency.values()).size();
        if (size == 1) {
            return "YES";
        } 
        for (Integer value : frequency.values()) {
            freqCount.put(value, freqCount.getOrDefault(value, 0) + 1);
        }
        if (freqCount.size() == 2) {
            if (Collections.min(freqCount.keySet()) == 1 && freqCount.get(Collections.min(freqCount.keySet())) == 1) {
                return "YES";
            } else if (Collections.max(freqCount.keySet())-Collections.min(freqCount.keySet())==1 && freqCount.get(Collections.max(freqCount.keySet())) == 1) {
                return "YES";
            } else {
                return "NO";
            }
        }
        return "NO";
    }