int timer = 100;
void setup() {

  for(int thisPin = 2; thisPin < 11; thisPin++) {
    digitalWrite(thisPin, HIGH);
    delay(timer);

    digitalWrite(thisPin, LOW);
  }
}

void loop() {
  for(int thisPin = 2; thisPin<11; thisPin++) {
    digitalWrite(thisPin, HIGH);
    delay(timer);
    digitalWrite(thisPin, LOW);
  }

  for(int thisPin = 10; thisPin >=2; thisPin--) {
    digitalWrite(thisPin,HIGH);

    delay(timer);

    digitalWrite(thisPin, LOW);
  }
}