Count Unique Vowel Strings

import java.util.*;

class Solution {
    public int vowelCount(String s) {
        int a = 0;
        HashMap<Character, Integer> hm = new HashMap<>();
        for (char ch : s.toCharArray()) {
            if (ch == 'a' || ch == 'e' || ch == 'i' || ch == 'o' || ch == 'u')
                hm.put(ch, hm.getOrDefault(ch, 0) + 1);
        }

        if (hm.size() == 0)
            return a;

        a = fact(hm.size());
        for (char ch : hm.keySet())
            a = a * hm.get(ch);

        return a;
    }

    int fact(int n) {
        if (n == 0 || n == 1)
            return 1;
        else
            return n * fact(n - 1);
    }

}