int led_pin[] = {2,3,4,5,6};
int num_led = sizeof(led_pin)/sizeof(int);

void setup() {
  // put your setup code here, to run once:
  Serial.begin(115200);
  for (int i=0;i<num_led;i++){
    pinMode(led_pin[i], OUTPUT);
    digitalWrite(led_pin[i], LOW);
  }
}

void loop() {
  // put your main code here, to run repeatedly:
  for (int i = 0;i<num_led;i++){
    digitalWrite(led_pin[i], HIGH);
    delay(500);
  }
  for (int n = num_led-1;n>=0;n--){
    digitalWrite(led_pin[n], LOW);
    delay(500);
  }
  delay(500);
}