1417. Reformat The String

class Solution {
    public String reformat(String s) {
        List<Character> letters = new LinkedList<>();
        List<Character> digits = new LinkedList<>();
        StringBuilder sb = new StringBuilder();
        
        for (char ch : s.toCharArray()) {
            if(Character.isDigit(ch)) {
                digits.add(ch);
            } 
            if(Character.isLetter(ch)) {
                letters.add(ch);
            } 
        }

        if ((Math.abs(letters.size() - digits.size()) > 1)) return "";

        boolean isLetter = letters.size() >= digits.size();

        int i = 0, j = 0;
        while (i < letters.size() || j < digits.size()) {
            if (isLetter && i < letters.size()) {
                sb.append(letters.get(i));
                i++;
            } else if (!isLetter && j < digits.size()) {
                sb.append(digits.get(j));
                j++;
            }
            isLetter = !isLetter;
        }

        return sb.toString();
    }
}