#include<bits/stdc++.h>
using namespace std;
int main(){
    string str;
    getline(cin,str);
    for(int i=0;i<str.size();i++)
    {
        if(str[i]=='a'||str[i]=='e'||str[i]=='i'||str[i]=='o'||str[i]=='u')
        {
            printf("%c",str[i]-32);
			continue;
        }
        else if(str[i]<='Z'&&str[i]>='A')
        {
			if(str[i]!='A'&&str[i]!='E'&&str[i]!='I'&&str[i]!='O'&&str[i]!='U')
            printf("%c",str[i]+32);
			else printf("%c",str[i]);
			continue;
        }
        printf("%c",str[i]);
    }
    
    return 0;
}

已AC，放心食用

#include <bits/stdc++.h>
using namespace std;
int main()
{
    string s;
    char c,d;
    getline(cin,s);
    for (int i = 0;i < s.size();i++)
    {
        c = s[i];
        if (c == 'a' || c == 'e' || c == 'i' || c == 'o' || c == 'u')
        {
            d = c - 32;
            cout << d;
            continue;
        }
        if (c >= 'B' && c <= 'Z' && c != 'E' && c != 'I' && c != 'O' && c != 'U')
        {
            d = c + 32;
            cout << d;
            continue;
        }
        cout << c;
    }
}