反向操作即可

#include<iostream>
#include<cstring>
#include<algorithm>
using namespace std;
string s;
int main(){
    cin>>s;
    reverse(s.begin(),s.end());
    for(int i=0;i<s.size();i++){
        if('A'<=s[i]&&s[i]<='Z') s[i]+=32;
        else s[i]-=32;
    }
    for(int i=0;i<s.size();i++){
        if(s[i]=='x') s[i]='a';
        else if(s[i]=='y') s[i]='b';
        else if(s[i]=='z') s[i]='c';
        else if(s[i]=='X') s[i]='A';
        else if(s[i]=='Y') s[i]='B';
        else if(s[i]=='Z') s[i]='C';
        else s[i]+=3;
    }
    cout<<s;
}