const int ledCount = 10;    
const int delayTime = 500; 

void setup() {
  for (int i = 2; i < 1 + ledCount; i++) {
  pinMode(i, OUTPUT);
  }
}

void loop() {
  
  for (int i = 1; i < 2 + ledCount; i++) {
    digitalWrite(i, HIGH);
    delay(delayTime);       
    digitalWrite(i, LOW);  
  }

  
  for (int i = 1 + ledCount - 1; i >= 0; i--) {
    digitalWrite(i, HIGH); 
    delay(delayTime);       
    digitalWrite(i, LOW);  
  }
}