int led[]={0,1,2,3,4,5,6,7};
int i;
void setup() {
  for (i=0; i<=7; i++){
     pinMode(led[i], OUTPUT);
  }
}

void loop() {
  for (i=0; i<=7; i++){
    digitalWrite(led [i], LOW);
    delay (50);
  }

  for (i=8; i>=0; i--){
    digitalWrite(led [i], HIGH);
    delay (50);
  }
}