int leds[]={15,2,0,4,16,17};
void setup(){
  for(int i=0;i<6;i++){
    pinMode(leds[i],OUTPUT);
  }
}

void loop(){
  int i,j;
  for(i=0,j=5;i<=3,j>=2;i++,j--)
  {
    digitalWrite(leds[i],HIGH);
    digitalWrite(leds[j],HIGH);
    delay(10);
    digitalWrite(leds[i],LOW);
    digitalWrite(leds[j],LOW);
    delay(100);
  }
  for(i=2,j=3;i>=0,j<=5;i--,j++)
  {
    digitalWrite(leds[i],HIGH);
    digitalWrite(leds[j],HIGH);
    delay(10);
    digitalWrite(leds[i],LOW);
    digitalWrite(leds[j],LOW);
    delay(100);
  }
}