public class Solution {
    public boolean isPangram(String sentence) {
      if (sentence == null || sentence.isEmpty()){
            return false;
        }

        String normalizeString = sentence.toLowerCase();
        Set<Character> letters = new HashSet<>();

        for (int i = 0; i <normalizeString.length(); i++){
            char c = normalizeString.charAt(i);

            if (c >= 'a' && c <= 'z'){
                letters.add(c);
            }

        }
        
        return letters.size() == 26;
    }
}