void setup() {
  for (int i = 0; i < 20; i++) {
    pinMode(i, OUTPUT);
  }
}

void loop() {
int CODE1[] = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19};
switchLEDs(CODE1);

int CODE2[] = {19, 18, 17, 16, 15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0};
switchLEDs(CODE2);
}

void switchLEDs(int LED[]) {

  for (int i = 0; i < (sizeof(LED) / sizeof(LED[0])); i++) {
    digitalWrite(LED[i], HIGH);
    delay(100);               // wait for a pause
    digitalWrite(LED[i], LOW);
    delay(100);               // wait for a pause
  }
}