Split in Three

#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>

using namespace std;
using namespace __gnu_pbds;
 
#define fi first
#define se second
#define mp make_pair
#define pb push_back
#define fbo find_by_order
#define ook order_of_key
 
typedef long long ll;
typedef pair<int,int> ii;
typedef vector<int> vi;
typedef long double ld; 
typedef pair<ld,ld> state;
typedef tree<int, null_type, less<int>, rb_tree_tag, tree_order_statistics_node_update> pbds;

int cnt[222222];
int main()
{
	ios_base::sync_with_stdio(0); cin.tie(0);
	int n; cin>>n;
	if(n%3==1) {cout<<"IMPOSSIBLE\n"; return 0;}
	vi a[3];
	if(n%6==2)
	{
		a[0]={};
		a[1]={1};
		a[2]={2};
	}
	else if(n%6==3)
	{
		a[0]={1};
		a[1]={2};
		a[2]={3};
	}
	else if(n%6==5)
	{
		a[0]={1,3};
		a[1]={5};
		a[2]={2,4};
	}
	else
	{
		a[0]={2,4};
		a[1]={1,6};
		a[2]={3,5};
	}
	int smol = n%6;
	if(smol==0) smol=6;
	smol++;
	for(int i=smol;i<=n;i+=6)
	{
		a[0].pb(i); a[0].pb(i+5);
		a[1].pb(i+1); a[1].pb(i+4);
		a[2].pb(i+2); a[2].pb(i+3);
	}
	for(int i=0;i<3;i++)
	{
		for(int x:a[i]) cnt[x]=i+1;
	}
	for(int i=1;i<=n;i++)
	{
		cout<<cnt[i];
		if(i+1<=n) cout<<' ';
	}
	cout<<'\n';
}

Test 1

Group: 1, 2

Verdict: ACCEPTED

Test 2

Group: 1, 2

Verdict: ACCEPTED

Test 3

Group: 1, 2

Verdict: ACCEPTED

Test 4

Group: 1, 2

Verdict: ACCEPTED

Test 5

Group: 1, 2

Verdict: ACCEPTED

Test 6

Group: 1, 2

Verdict: ACCEPTED

Test 7

Group: 1, 2

Verdict: ACCEPTED

Test 8

Group: 1, 2

Verdict: ACCEPTED

Test 9

Group: 2

Verdict: ACCEPTED

Test 10

Group: 2

Verdict: ACCEPTED

Test 11

Group: 2

Verdict: ACCEPTED

Test 12

Group: 2

Verdict: ACCEPTED

Test 13

Group: 2

Verdict: ACCEPTED

Test 14

Group: 2

Verdict: ACCEPTED

Test 15

Group: 2

Verdict: ACCEPTED