Split in Three

#include <bits/stdc++.h>
#define pb push_back
#define mp make_pair
#define st first
#define nd second
#define pii pair<int,int>
#define N 2000005
#define INF 1e9+5
#define sp " "
#define nl "\n"
#define all(x) x.begin(),x.end()
#define fastio() ios_base::sync_with_stdio(0);cin.tie(0);
#define fori(i,l,r) for(int i=l;i<=r;i++)
#define forf(i,r) fori(i,1,r)
#define rof(i,r,l) for(int i=r;i>=l;i--)
#define ll long long
#define int ll
using namespace std;

int binpow(int x,int y){
    int ans=1;
    for(;y>0;x*=x,y>>=1){
        if(y&1)
            ans*=x;
    }
    return ans;
}

void solve(){
	int n;
	cin >> n;
	if(n==1){
        cout << 1;
        return;
	}
	if(n%3!=0&&(n+1)%3!=0){
        cout << "IMPOSSIBLE\n";
        return;
	}
	if(n==3){
        cout << "1 2 3";
        return;
	}
	int sum=(n+1)*n/2;
	sum/=3;
	vector<int> ans(n+1,0);
	if(sum%2==0){
        ans[sum/2-1]=2;
        ans[sum/2+1]=2;
        ans[sum/2+2]=1;
        ans[sum/2-3]=1;
        for(int i=1;i<=n;i++){
            if(ans[i])
                cout << ans[i] << sp;
            else
                cout << 3 << sp;
        }
	}
	else{
        ans[sum/2]=2;
        ans[sum/2+1]=2;
        ans[sum/2-2]=1;
        ans[sum/2+2]=1;
        for(int i=1;i<=n;i++){
            if(ans[i])
                cout << ans[i] << sp;
            else{
                cout << 3 << sp;
            }
        }
	}
}

int32_t main(){
	fastio()
	int t=1;
	//cin >> t;
	while(t--){
		solve();
	}
}

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: WRONG ANSWER

Test 10

Group: 2

Verdict: WRONG ANSWER

Test 11

Group: 2

Verdict: WRONG ANSWER

Test 12

Group: 2

Verdict: ACCEPTED

Test 13

Group: 2

Verdict: WRONG ANSWER

Test 14

Group: 2

Verdict: WRONG ANSWER

Test 15

Group: 2

Verdict: ACCEPTED