C语言编程：将26个英文字母按aAbBcC……zZ先小写后大写的顺序逐个打印出来?

#include <stdio.h>
main()
{
int i;
for(i=0;i<26;i++)
printf("%c%c",(97+i),(65+i));

}

//---------------------------------------------------------------------------

#include <stdio.h>
#include <ctype.h>

int main(void)
{
char a;
for (a='a'; a<='z'; a++) printf("%c%c",a,toupper(a));
return 0;
}
//---------------------------------------------------------------------------