int leds[] = {13,12,11,10,9,8,7,6,5,4,3,2};

void setup(){
  for (int i=0; i<12; i++){
    pinMode(leds[i], OUTPUT);}
}

void loop()
{
  for (int i=0; i<12;){
    digitalWrite(leds[i], HIGH);
    i++;
    delay(100);
  }
  for (int b=0; b<12;)
  {
    digitalWrite(leds[b], LOW);
    b++;
    delay(100);
  }
}