void setup() {
  for(int n=1;n<=7;n++){
    pinMode(n, OUTPUT);
  }
}

void loop() {
  for(int n=2;n<=6;n++){
    digitalWrite(n, HIGH);
    delay(100);
    digitalWrite(n, LOW);
    delay(100);
  }
   for(int n=6;n>=2;n--){
    digitalWrite(n, HIGH);
    delay(100);
    digitalWrite(n, LOW);
    delay(100);
  }
}