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;
}