习题：输入一个字符判断其大小写 并相互转换

# include <stdio.h>

int main(void)
{
 char a, A, B;

step1:
 printf("请输入单个字母:");  
 
 a = getchar();  
 
   fflush(stdin);  //清掉enter键

 if ('A' <= a && a <= 'Z' ) 
 {
  A = a + 32;   //将大写字母转为小写字母

  printf("该字母为大写：%c对应小写为：%c\n", a, A);
 }

 else if ('a' <= a && a <= 'z') 
 {
  B = a - 32;   //将小写字母转为大写字母
 
  printf("该字母为小写：%c,对应大写为：%c\n",a, B);
 }

 else
 {
  printf("人才，你输入的不是字母!\n");
 }

goto step1;
 return 0;
}