直接用字典法,简单快捷

#include <iostream>
using namespace std;
int main(){
    string str1, str2;
    cin >> str1 >> str2;
    string a("abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789");
    string b("BCDEFGHIJKLMNOPQRSTUVWXYZAbcdefghijklmnopqrstuvwxyza1234567890");
    for(int i = 0; i < str1.size(); ++i)
        str1[i] = b[a.find_last_of(str1[i])];
    for(int i = 0; i < str2.size(); ++i)
        str2[i] = a[b.find_last_of(str2[i])];
    cout << str1 << endl << str2;
}