int led[13] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13} ;

void setup() {
  // put your setup code here, to run once:
for (int i = 0; i < 13; i++){
pinMode(led[i], OUTPUT);
}
}

void loop() {
  // put your main code here, to run repeatedly:
  for (int i = 0; i < 13; i++){
digitalWrite(led[i], HIGH);
delay(100);
digitalWrite(led[i], LOW);
delay(100);
}
  for (int i = 12; i > 0; i--){
    digitalWrite(led[i], HIGH);
    delay(100);
    digitalWrite(led[i], LOW);
    delay(100);
  }
}