alphabet_vector[26] = {0}
most_common_char_counter = 0

for (input_counter = 0; input_conuter < 20; input_conuter++)
{
    c = getchar()
    alphabet_vector[c - 'a']++

    if (alphabet_vector[c - 'a'] > most_common_char_count)
    {
        most_common_char_count = alphabet_vector[c - 'a']
        most_common_char = c
    }
}

print most common char is most_common_char