Split in Three

# include <iostream>
# include <cmath>
# include <algorithm>
# include <stdio.h>
# include <cstdint>
# include <cstring>
# include <string>
# include <cstdlib>
# include <vector>
# include <bitset>
# include <map>
# include <queue>
# include <ctime>
# include <stack>
# include <set>
# include <list>
# include <random>
# include <deque>
# include <functional>
# include <iomanip>
# include <sstream>
# include <fstream>
# include <complex>
# include <numeric>
# include <immintrin.h>
# include <cassert>
# include <array>
# include <tuple>
# include <unordered_set>
# include <unordered_map>
 
//#include<bits/stdc++.h>
 
 
#define LL long long
#define LD long double
#define pb push_back
#define mp make_pair
#define ss second
#define ff first
#define opt ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL)
#define _sz(x) (LL)x.size()
#define _all(x) x.begin(),x.end()
#define el '\n'
#define eb emplace_back
 
using namespace std;
const LL N = 1, INF = 1e18;
 
void solve() {
    LL n = 0;
    cin >> n;
    if(n*(n+1)/2%3 != 0 || n < 3)
    {
        cout << "IMPOSSIBLE" << el;
        return;
    }
    LL ans[n+27];
    for(LL i = 0; i <= n; i++)
        ans[i] = 3;
    LL sum = n*(n+1)/2/3 -1;
    for(LL i = n; i >= 1; i--)
    {
        if(sum - i >= 0 && ans[i] == 3)
        {
            ans[i] = 1;
            sum -= i;
        }
    }
    if(sum != 0)
        exit(-1);
    sum = n*(n+1)/2/3;
    for(LL i = n; i >= 1; i--)
    {
        if(sum - i >= 0 && ans[i] == 3)
        {
            ans[i] = 2;
            sum -= i;
        }
    }
    if(sum != 0)
        exit(-1);
    for(LL i = 1; i <= n; i++)
        cout << ans[i] << " ";
    cout << el;
}
 
int main()
{
    opt;
    LL q = 1;
    //cin >> q;
    while(q--)
        solve();
    return 0;
}

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