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

void setup() {
  // Set each pin as an output
  for (int i = 0; i < numLeds; i++) {
    pinMode(ledPins[i], OUTPUT);
  }
}

void loop() {
  // Turn on LEDs in the specified array order
  for (int i = 0; i < numLeds; i++) {
    digitalWrite(ledPins[i], HIGH);
    delay(100); // Wait 100ms
  }

  // Turn off LEDs in reverse order
  for (int i = numLeds - 1; i >= 0; i--) {
    digitalWrite(ledPins[i], LOW);
    delay(100); // Wait 100ms
  }
}