C++ :

#include<bits/stdc++.h>
using namespace std;
int n,m,a[10005],ans,x,sum;
int main(){
	cin>>n>>m;
	while(m--)
	{
		cin>>x;
		if(a[x]==0) sum++;
		a[x]++;
		ans=max(ans,a[x]);
	}
	cout<<sum<<endl;
	for(int i=1;i<=n;i++)
		if(a[i]==ans) cout<<i<<" ";
	return 0;
}