const char* LETTERS = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
const char* HEX = "0123456789ABCDEF"

char c;
const int LETTERS_LEN = strlen(LETTERS);
for (int i = 0; i < N; i++) {
    c = LETTERS[i % LETTERS_LEN]
    // place c in your triangle
}