int leds[]= {6,7,8,9,10,11,12,13};
byte pinCount= 8;
void setup() {
  // put your setup code here, to run once:
  for (int i=0; i<pinCount; i++){
    pinMode(leds[i], OUTPUT);
  }
}

void loop() {
  // put your main code here, to run repeatedly:
  for(int i=0; i<pinCount; i++){
    digitalWrite(leds[i], LOW);
    delay(100);
  }
  for(int i=pinCount; i>=0; i--){
    digitalWrite(leds[i], HIGH);
    delay(100);
  }
}