int ledPins[] = {2, 3, 4, 5, 6, 7};
int ledCount = 6;

void setup() {
  // Set all LED pins as output
  for (int i = 0; i < ledCount; i++) {
    pinMode(ledPins[i], OUTPUT);
  }
}

void loop() {
  // Forward wave
  for (int i = 0; i < ledCount; i++) {
    digitalWrite(ledPins[i], HIGH);
    delay(100);
    digitalWrite(ledPins[i], LOW);
  }

  // Reverse wave
  for (int i = ledCount - 1; i >= 0; i--) {
    digitalWrite(ledPins[i], HIGH);
    delay(100);
    digitalWrite(ledPins[i], LOW);
  }

  // All blink together
  for (int j = 0; j < 3; j++) {
    for (int i = 0; i < ledCount; i++) {
      digitalWrite(ledPins[i], HIGH);
    }
    delay(200);
    for (int i = 0; i < ledCount; i++) {
      digitalWrite(ledPins[i], LOW);
    }
    delay(200);
  }
}