可以实现东西红灯南北绿灯——东西红灯亮南北转黄灯——东西绿灯南北红灯——东西转黄灯南北红灯亮的转换（初始状态为四方红灯）。-North and South can achieve something red light green light- north and south turn things red light yellow- green north-south red things- things that turn yellow north-south red light conve