int timer = 500;             
int LEDPins[] = {13, 12, 8, 4, 2};       // an array of declared pin numbers on the board  
int countOFpin = 5;           // the number of arrays  
void setup()   
{  
  // we have declared an array to intialize the LED pins as OUTPUT  
  for (int PIN = 0; PIN < countOFpin; PIN= PIN + 1)   
{  
    pinMode(LEDPins[PIN], OUTPUT);  
  }  
}  
void loop()  
 {  
  // loop starting from the lowest pin in the array to the highest:  
  for (int PIN = 0; PIN < countOFpin; PIN++) {  
    // turns the pin ON:  
    digitalWrite(LEDPins[PIN], HIGH);  
    delay(timer);  
    // turnS the pin OFF:  
    digitalWrite(LEDPins[PIN], LOW);  
  }  
  // loop from the highest pin in the array to the lowest:   
  // It means the LEDs will light in the reverse direction as used above  
  for (int PIN = countOFpin - 1; PIN >= 0; PIN--)  
 {   
    digitalWrite(LEDPins[PIN], HIGH);  
    delay(timer);  
    digitalWrite(LEDPins[PIN], LOW);  
    // We can also specify the time inside the delay( ) instead of the delcaring the timer  
  }  
}