16行，应该是最简单最直观最唐的做法了

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

int main(){
	ios:: sync_with_stdio(0);
	cin.tie(0);
	string s;
	cin>>s;
	sort(s.begin(),s.end(),[](char aa,char bb){
		if(aa < '0' or aa > '9')  return (bool)0;
		if(bb < '0' or bb > '9')  return (bool)1;
		return aa > bb;
	});
	for(int i = 0; i < s.size() and '0' <= s[i] and s[i] <= '9'; ++i){
		cout<<s[i];	
	}
	return 0;
}