题解string+sort

#include<bits/stdc++.h>
using namespace std;

int main(){
    string s,lowers,uppers,numbers,others;
    cin>>s;
    for (char i:s){
        if (isupper(i)) uppers+=i;
        if (islower(i)) lowers+=i;
        if (isdigit(i)) numbers+=i;
        if (ispunct(i)) others+=i;
    }
    sort(lowers.begin(),lowers.end());
    sort(uppers.begin(),uppers.end());
    sort(numbers.begin(),numbers.end());
    sort(others.begin(),others.end());
    cout<<lowers<<uppers<<numbers<<others;
    return 0;
}