const int buttonPin = 2;
const int redLED = 6;
const int yellowLED = 5;
const int greenLED = 4;
int LEDCounter = 0;

void setup() {
  Serial.begin(9600);
  
  // Set the button pin as input
  pinMode(buttonPin, INPUT);
  pinMode(redLED, OUTPUT);
  pinMode(yellowLED, OUTPUT);
  pinMode(greenLED, OUTPUT);

  digitalWrite(redLED, HIGH);
  Serial.println(LEDCounter);
  attachInterrupt(digitalPinToInterrupt(buttonPin), buttonPressed, FALLING);
}

void loop() {

}

void buttonPressed() {
  incrementLEDs();
}

void incrementLEDs() {
  if (LEDCounter == 2) {
    LEDCounter = 0;
  }
  else {
    LEDCounter += 1;
  }

  if (LEDCounter == 0) {
    digitalWrite(redLED, HIGH);
    digitalWrite(yellowLED, LOW);
    digitalWrite(greenLED, LOW);
  }
  else if (LEDCounter == 1) {
    digitalWrite(redLED, LOW);
    digitalWrite(yellowLED, HIGH);
    digitalWrite(greenLED, LOW);
  }
  else if (LEDCounter == 2) {
    digitalWrite(redLED, LOW);
    digitalWrite(yellowLED, LOW);
    digitalWrite(greenLED, HIGH);
  }

  Serial.println(LEDCounter);
}