int timer = 35; // Delay time between steps

void setup() {
  // Initialize all pins as outputs using a loop
  for (int thisPin = 2; thisPin <= 10; thisPin++) {
    pinMode(thisPin, OUTPUT);
  }
}

void loop() {
  // Turn LEDs on one by one
  for (int thisPin = 2; thisPin <= 10; thisPin++) {
    digitalWrite(thisPin, HIGH);
    delay(timer);
    digitalWrite(thisPin, LOW);
  }

  // Turn LEDs off one by one in reverse
  for (int thisPin = 10; thisPin >= 2; thisPin--) {
    digitalWrite(thisPin, HIGH);
    delay(timer);
    digitalWrite(thisPin, LOW);
  }
}