//byte ledPins[] = {15, 2, 0, 4, 5, 18, 19, 21, 22 ,23};
byte ledPins[] = {42, 41, 40, 39, 38, 37, 36, 35, 34 ,33};
int ledCount;

void setup() {
  ledCount = sizeof(ledPins);
  for (int i = 0; i < ledCount; i++) {
    pinMode(ledPins[i], OUTPUT);
  }
  Serial.begin(115200);
}

void loop() {
  // One way
  for (int i = 0; i < ledCount; i++) {
    digitalWrite(ledPins[i], HIGH);
    delay(50);
    digitalWrite(ledPins[i], LOW);
  }

  delay(50);

  // And reverse
  for (int i = ledCount - 1; i >= 0; i--) {
    Serial.println(i);
    digitalWrite(ledPins[i], HIGH);
    delay(50);
    digitalWrite(ledPins[i], LOW);
  }

  delay(1000);
}