题解（vector）

#include <iostream>
#include <cstdio>
#include <algorithm>
#include <vector>
using namespace std;
string s;
vector <char> a, b, c, d;
void print(vector <char> a){
	for(int i = 0; i < a.size(); i++) cout << a[i];
}
int main(){
	cin >> s;
	for(int i = 0; s[i] != '\0'; i++){
		if('a' <= s[i] && s[i] <= 'z') a.push_back(s[i]);
		else if('A' <= s[i] && s[i] <= 'Z') b.push_back(s[i]);
		else if(isdigit(s[i])) c.push_back(s[i]);
		else d.push_back(s[i]); 
	}sort(a.begin(), a.end()), sort(b.begin(), b.end());
	sort(c.begin(), c.end()), sort(d.begin(), d.end());
	print(a), print(b), print(c), print(d);

	return 0;
}