程序代码：

#include<stdio.h>
enum TRAFFIC_LED{RED,GREEN,YELLOW};
void wait1sec(void);

void main()
{
    enum TRAFFIC_LED led;
    char display[3][4]={"R","G","Y"};
    int timers;
    led=RED;
    while(1)
    {
        if(led==RED)
        {
            printf("%c",'\r');
            printf("%s",display[led]);
            timers=30;
            do{
                printf("%3d",timers);
                timers--;
                printf("%c%c%c",'\b','\b','\b');
                wait1sec();
            }while(timers>0);
            led=GREEN;
        }
        else if(led==GREEN)
        {
            printf("%c",'\r');
            printf("%s",display[led]);
            timers=30;
            do{
                printf("%3d",timers);
                timers--;
                printf("%c%c%c",'\b','\b','\b');
                wait1sec();
            }while(timers>0);
            led=YELLOW;
        }
        else
        {
            printf("%c",'\r');
            printf("%s",display[led]);
            timers=5;
            do{
                printf("%3d",timers);
                timers--;
                printf("%c%c%c",'\b','\b','\b');
                wait1sec();
            }while(timers>0);
            led=RED;
    }
}
void wait1sec(void)
{
    int n;
    n=1;
    while(n<10000000)
        n++;
}