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

void loop() {
  for(int i=0;i<=7;i++){
    digitalWrite(led[i],1);
    delay(500);
  }
  for(int i=7;i>0;i--){
    digitalWrite(led[i],0);
    delay(500);
  }
}