byte ledPins[] = {15, 2, 4, 5, 18, 19, 21, 12, 26, 25};

int jj;

void setup()
{
  jj = sizeof(ledPins);

  for (int i = 0; i < jj; i++)
  {
    pinMode(ledPins[i], OUTPUT);
  }
}

void loop()
{
  for (int i = 0; i < jj; i++)
  {
    digitalWrite(ledPins[i], HIGH);
    delay(100);
    digitalWrite(ledPins[i], LOW);
    delay(50);
  }
  for (int i = jj - 1; i > -1; i--)
  {
    digitalWrite(ledPins[i], HIGH);
    delay(100);
    digitalWrite(ledPins[i], LOW);
    delay(50);
  }
}