public class Solution {
    public int commonChars(String a, String b) {
        // Escribe tu solución aquí
        int contador = 0;
        List<Character> letters = new ArrayList<Character>();
        for (char character1 : a.toCharArray()) {
            if (!letters.contains(character1)) {
                letters.add(character1);
                for (char character2 : b.toCharArray()) {
                    if (Character.toLowerCase(character1) == Character.toLowerCase(character2)) contador++;
                }
            }
        }

        return contador;
    }
}