public static String isValid(String s) {
        // Write your code here
        HashMap<Character, Integer> letterCountMap = new HashMap<>();
        HashMap<Integer, Integer> occMap = new HashMap<>();
        
        // count letter
        for(Character ch : s.toCharArray()) {
            letterCountMap.put(ch, letterCountMap.getOrDefault(ch, 0)+1);
        }
        
        // count of occurences of the occurences of the letter
        for(Map.Entry<Character, Integer> entry : letterCountMap.entrySet()) {
            occMap.put(entry.getValue(), occMap.getOrDefault(entry.getValue(), 0)+1);
        }
        // if all equal
        if (occMap.size() == 1)
            return "YES";
            
        // find largest count & smallest count
        // don't really need to keep track of smallest & largest
        // only need to store the 2 key value into variable for comparing later
        int largest_key = Integer.MIN_VALUE;
        int smallest_key = Integer.MAX_VALUE;
        int largest_value = 0;
        int smallest_value = 0;
        for(Map.Entry<Integer, Integer> entry : occMap.entrySet()) {
            if (entry.getValue() > largest_key) {
                largest_key = entry.getKey();
                largest_value = entry.getValue();
            }
            if (entry.getValue() < smallest_key) {
                smallest_key = entry.getKey();
                smallest_value = entry.getValue();
            }
        }
        
        // System.out.println("count map: " + letterCountMap);
        // System.out.println("occ map: " + occMap);
        // System.out.println("largest: " + largest_key);
        // System.out.println("smallest: " + smallest_key);

        if (occMap.size() == 2) {
            if (occMap.get(smallest_key) == 1 || occMap.get(largest_key) == 1) 
                if (smallest_key == 1 || largest_key == 1)
                    return "YES";
                else if (Math.abs(smallest_key-largest_key) == 1)
                    return "YES";
        }
            
        return "NO";
    }