int pins[6] = {19, 18, 17, 16, 25, 26};

void setup() {
  Serial.begin(115200);
  for(int i = 0; i < 6; i++) {
    pinMode(pins[i], OUTPUT);
  }
}

void loop() {
  for(int i = 0; i < 6; i++){
   digitalWrite(pins[i], HIGH);
   delay(100);
   digitalWrite(pins[i], LOW);
  }

  for(int i = 5; i > 0; i--){
    digitalWrite(pins[i], HIGH);
    delay(100);
    digitalWrite(pins[i], LOW);
  }
}