题解（大小写转换）

#include<iostream>
using namespace std;

int main(){
	string s;
    cin>>s;
    for (int i=0;i<s.size();i++) s[i]=tolower(s[i]);
    for (int i=0;i<s.size();i++){
    	if (s[i]=='a' || s[i]=='c' || s[i]=='g' || s[i]=='o')
            s[i]=toupper(s[i]);
    }
    cout<<s;
    return 0;
}