int led[6] = {3,5,6,9,10,11};
int a = 0 , b = 5;

void setup() { 
Serial.begin(115200); 
for (int i = a; i <= b; i++) {

pinMode(led[i], OUTPUT); 

 }
}

void loop() {

  for (int i = a; i <= b; i++) {

  analogWrite(led[i], HIGH); 
   Serial.println(led[i]);
  delay(700);
}

  for (int i = b; i >= a; i--) {

  analogWrite(led[i], LOW); 
   Serial.println(led[i]);
  delay(700);
}

}