c语言中选择结构

void swap(char *a, char *b)
{
char tmp;
if (!a || !b) return;

if (*a < *b)
{
tmp = *a;
*a = *b;
*b = tmp;
}
}

int main (int argc, char *argv[])
{
char a, b;
printf("input character a: ")
scanf("%c", &a);
printf("input character b: ");
scanf("%c", &b);
swap(&a, &b);
printf("a: %c, b: %c\n", a, b);
}

用一个中间变量，
if(a{
t=a;
b=a;
b=t;
}
交换就行了

void fun(char a,char b)
{
char c;
if(a {
c=a;
a=b;
b=c;
printf("%c %c\n",a,b);
}
}
int main()
{
char x,y;
printf("输入两个字符%c%c",&x,&y);
fun(x,y);
getch();
}