把一个字符串中所有出现的大写字母都替换成小写字母，同时把小写字母替换成大写字母

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#include<stdio.h>
#include<string.h>
int main()
{
char str[90];
while(gets(str)!=NULL)
{
int i;
for(i=0;str[i]!='\0';i++)
{
if(str[i]>='A'&&str[i]<='Z')
str[i]+=32;
else if(str[i]>='a'&&str[i]<='z')
str[i]-=32;
}
puts(str);
printf("\n");
}
return 0;
}