Why doesn't my Pangram solution output correctly?

Question

The question is as follows: A pangram is a string that contains every letter of the alphabet. Given a sentence determine whether it is a pangram in the English alphabet. Ignore case. Return either pangram or not pangram as appropriate.

My code works for all input except "qmExzBIJmdELxyOFWv LOCmefk TwPhargKSPEqSxzveiun", for which it returns "not pangram", even though the correct answer is "pangram." Does anybody have any ideas as to why my code is outputting the incorrect solution?

public static String pangrams(String s) {

    Hashtable<Character, Integer> alpha = new Hashtable<Character, Integer>();
    s = s.toLowerCase();
    for (int i = 0; i < s.length()-1; i++){
        if (alpha.get(s.charAt(i)) != null){
            int value = alpha.get(s.charAt(i));
            alpha.put(s.charAt(i), value + 1);
        }
        else{
            alpha.put(s.charAt(i), 1);
        }
    }

  
    if (alpha.size() != 27){
        return "not pangram";
    }
    else{
        return "pangram";
    }
        
}

Answer 1

As per your definition of pangram you might want to consider the following:

1. Use a Set instead of a Map
2. As far as I know, English has 26 letters
3. Make sure you don't mistakenly add digits or symbols to your set

Given the three above statements I would slightly change your code to reflect that. That would, more or less, look something like this:

public static String pangrams(String s) {
    Set<Character> alpha = new HashSet<>();
    s = s.toLowerCase();
    for (int i = 0; i < s.length(); i++){
        char c = s.charAt(i);
        if (Character.isLetter(c)){
            alpha.add(c);
        }
    }
    if (alpha.size() == 26){
        return "pangram";
    }
    return "not pangram";
}