int ledPins[] = {2, 3, 4, 5, 6, 7, 8, 9};
int numLeds = 8;

void setup() {
  // Set all LED pins to OUTPUT
  for (int i = 0; i < numLeds; i++) {
    pinMode(ledPins[i], OUTPUT);
  }
}

void loop() {
  // Go forward
  int i = 0;
  while (i < numLeds) {
    digitalWrite(ledPins[i], HIGH);
    delay(100);
    digitalWrite(ledPins[i], LOW);
    i++; // Go to next LED
  }

  // Go backward
  i = numLeds - 2; // Start from second last LED
  while (i > 0) {
    digitalWrite(ledPins[i], HIGH);
    delay(100);
    digitalWrite(ledPins[i], LOW);
    i--; // Go to previous LED
  }
}