#define DELAY 400

byte leds[] = {4, 5, 6, 7, 8, 9, 10, 11, 12, 13};
int direction = 1;
int currentLed = 0;
unsigned long changeTime;

void setup() {
  // put your setup code here, to run once:

  for (int i; i < 10; i++) {
    pinMode(leds[i], OUTPUT);
  }
  changeTime = millis();
}

void loop() {
  // put your main code here, to run repeatedly:
  if (millis() - changeTime > DELAY) {
    changeLED();
    changeTime = millis();
  }

}

void changeLED(){
  for (int i; i<10; i++){
    digitalWrite(leds[i], LOW);
  }
  
  digitalWrite(leds[currentLed], HIGH);
  currentLed += direction;
  if (currentLed == 0) {direction = 1;}
  if (currentLed == 9) {direction = -1;}
}