在屏幕上输出以下图案： * *** ***** ******* ********* *********** ************* *********** *********

int main()
{
int a, b;    
a = 1;
while (a <= 7)
{
b = 1;
while (b <= a)
{
printf("*");
b ++;
}
printf("\n");
a ++;
}
while (a > 7)
{
b = 7;
while (b >= a)
{
printf("*");
b --;
}
printf("\n");
a ++;
}
}