#include<bits/stdc++.h>
using namespace std;
int main(){
	char a;
	cin>>a;
	int b=0;
	int y=(int)a;
	char s;
	char d;
	if(y<=122 and y>=97){
		y=y-32;
		s=(char)y;
		cout<<s;
	} 
	if(a>=65 and a<=96){
		y=y+32;
		d=(char)y;
		cout<<d;
	}
	return 0;
} 

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