C :

#include <stdio.h>
int main(){
    char x,y;   
    x=getchar();  //输入字母，并且赋给x
    if(65<=x && x<=90){  //大写字母ASCII码范围为：65A~90Z，判断为大写执行代码
        y=x+32;  //A为65，a为97相差32，加32变为小写
        putchar(y);  //输出获得的小写字母
        putchar('\n');  //换行
    }
    else if(97<=x && x<=122){  //小写字母ASCII码范围为：97a~122z，判断为小写执行代码
        y=x-32;  //A为65，a为97相差32，减32变为大写
        putchar(y);  //输出获得的大写字母
        putchar('\n');  //换行
    }
    else{
        printf("输入有误！");  //输出提示语句，提醒输入错误
    }
return 0;
}

C++ :

#include<iostream>
using namespace std;

char ch;
int main(){
	//freopen("change.in","r",stdin);
	//freopen("change.out","w",stdout);
	cin >> ch;
	if (ch>='A'&&ch<='Z'){
		cout << (char)(ch+32);
	}
	if (ch>='a'&&ch<='z'){
		cout << (char)(ch-32);
	}
	return 0;
}

Python :

# coding=utf-8
da=['A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z']
xi=['a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z']
a=input()
for i in range(0,26):
    if(a==da[i]):
        print(xi[i])
        break
    if(a==xi[i]):
        print(da[i])
        break

一道可以把人淹死的水题

#include<bits/stdc++.h>
using namespace std;int main(){char a;cin>>a;if(a>='A'&&a<='Z') a+=32;else a-=32;cout<<a;return 0;}

water 题，判断下输入进去的是大写还是小写就完了。

#include<bits/stdc++.h>
using namespace std;
int main()
{
	char a;
	cin>>a;
	if(a>='A'&&a<='Z')
	{
		a+=32;
	}
	else
	{
		a-=32;
	}
	cout<<a;
	return 0;
}

因为只有字母，那么大于'a'的都是小写字母。

#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 c;
	cin>>c;
	if(islower(c))//如果是小写
      cout<<char(c-32);//转大写
	else cout<<char(c+32);//否则转小写
}