// Qiu Xinjian 1520213503
int timer = 100; 
void setup() { 
  for (int thisPin = 16; thisPin < 20; thisPin++) {
    pinMode(thisPin, OUTPUT);
  }
}
void loop() { // loop from the lowest pin to the highest:
  for (int thisPin = 16; thisPin < 20; thisPin++) { 
    digitalWrite(thisPin, HIGH);
    delay(timer); 
    digitalWrite(thisPin, LOW);
  }
  for (int thisPin = 19; thisPin >= 16; thisPin--) {
    digitalWrite(thisPin, HIGH);
    delay(timer); 
    digitalWrite(thisPin, LOW);
  }
}