void setup() {
  // put your setup code here, to run once:
  pinMode(2, OUTPUT);
  pinMode(3, OUTPUT);
  pinMode(4, OUTPUT);
  pinMode(5, OUTPUT);
  pinMode(6, OUTPUT);
  pinMode(7, OUTPUT);
}

void loop() {
  const int flash = 2, glight = 2000, ylight = 500;
  // 紅燈時間 = 綠燈時長+ 黃燈時長*閃爍次數-最後黃燈不亮時間
  const int rlight = glight + ylight * flash * 2 - ylight;

  int i;
  // put your main code here, to run repeatedly:
  digitalWrite(4, HIGH);
  digitalWrite(5, HIGH);
  delay(glight);
  digitalWrite(5, LOW);

  for (i = 0; i < flash; i++) {
    digitalWrite(6, HIGH);
    delay(ylight);
    digitalWrite(6, LOW);
    if (i < flash - 1)
      delay(ylight);
  }
  digitalWrite(6, LOW);

  digitalWrite(4, LOW);
  digitalWrite(7, HIGH);

  digitalWrite(2, HIGH);
  delay(glight);
  digitalWrite(2, LOW);

  for (i = 0; i < flash; i++) {
    digitalWrite(3, HIGH);
    delay(ylight);
    digitalWrite(3, LOW);
    if (i < flash - 1)
      delay(ylight);
  }
  digitalWrite(3, LOW);

  // delay(rlight);
  digitalWrite(7, LOW);
}