int timer = 35; // Delay time between steps

void setup() {
  // Initialize pins as outputs
  pinMode(2, OUTPUT);
  pinMode(3, OUTPUT);
  pinMode(4, OUTPUT);
  pinMode(5, OUTPUT);
  pinMode(6, OUTPUT);
  pinMode(7, OUTPUT);
  pinMode(8, OUTPUT);
  pinMode(9, OUTPUT);
  pinMode(10, OUTPUT);
}

void loop() {
  // Turn LEDs on one by one
  digitalWrite(2, HIGH); delay(timer); digitalWrite(2, LOW);
  digitalWrite(3, HIGH); delay(timer); digitalWrite(3, LOW);
  digitalWrite(4, HIGH); delay(timer); digitalWrite(4, LOW);
  digitalWrite(5, HIGH); delay(timer); digitalWrite(5, LOW);
  digitalWrite(6, HIGH); delay(timer); digitalWrite(6, LOW);
  digitalWrite(7, HIGH); delay(timer); digitalWrite(7, LOW);
  digitalWrite(8, HIGH); delay(timer); digitalWrite(8, LOW);
  digitalWrite(9, HIGH); delay(timer); digitalWrite(9, LOW);
  digitalWrite(10, HIGH); delay(timer); digitalWrite(10, LOW);

  // Turn LEDs off one by one in reverse
  digitalWrite(10, HIGH); delay(timer); digitalWrite(10, LOW);
  digitalWrite(9, HIGH); delay(timer); digitalWrite(9, LOW);
  digitalWrite(8, HIGH); delay(timer); digitalWrite(8, LOW);
  digitalWrite(7, HIGH); delay(timer); digitalWrite(7, LOW);
  digitalWrite(6, HIGH); delay(timer); digitalWrite(6, LOW);
  digitalWrite(5, HIGH); delay(timer); digitalWrite(5, LOW);
  digitalWrite(4, HIGH); delay(timer); digitalWrite(4, LOW);
  digitalWrite(3, HIGH); delay(timer); digitalWrite(3, LOW);
  digitalWrite(2, HIGH); delay(timer); digitalWrite(2, LOW);
}