int myPins[] = {3, 5, 9, 12};


void setup() {

  for (int i = 0; i < sizeof(myPins); i++) {
    pinMode(myPins[i], OUTPUT);
  }

}

void loop() {

  for (int i = 0; i < sizeof(myPins)/sizeof(myPins[0]); i++) {
    digitalWrite(myPins[i], HIGH);
    delay(500);
  }


  for (int i = sizeof(myPins)/sizeof(myPins[0]); i <= sizeof(myPins)/sizeof(myPins[0]); i--) {
    digitalWrite(myPins[i], LOW);
    delay(500);
  }

}