直接判断输出。

#include<bits/stdc++.h>
#define int long long
#define INF 0x3f3f3f
using namespace std;
char c;
signed main(){
	cin>>c;
	if(c>='a')cout<<char(c-32);
	else cout<<char(c+32);
	return 0;
}

#include<bits/stdc++.h>
using namespace std;
signed main(){
	char a;
	cin>>a;
	if(isupper(a))cout<<char(a+32);
	else cout<<char(a-32);
}