Split in Three

input/code.cpp: In function 'void gen(int, int, std::vector<bool>&)':
input/code.cpp:42:13: warning: this 'for' clause does not guard... [-Wmisleading-indentation]
             for(int i=1;i<=n;i++) cout<<a[i]<<" "; cout<<endl;
             ^~~
input/code.cpp:42:52: note: ...this statement, but the latter is misleadingly indented as if it were guarded by the 'for'
             for(int i=1;i<=n;i++) cout<<a[i]<<" "; cout<<endl;
                                                    ^~~~

#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
#define endl '\n'
#define INF 1e9+7
#define all(x) x.begin(),x.end()
using namespace std;
using namespace __gnu_pbds;
using ll=long long;
using pii=pair<int,int>;
using ppi=pair<int,pii>;
using oset=tree<int,null_type,less<int>,rb_tree_tag,tree_order_statistics_node_update>;

template<typename T>
void _print(vector<T> x) {cerr<<"{"; for(auto e:x) cerr<<e<<","; cerr<<"}";}
void _print(pii x) {cerr<<"{"<<x.first<<","<<x.second<<"}";}
template<typename T>
void _print(T x) {cerr<<x;}

void dbg() {cerr<<endl;}
template<typename Head,typename... Tail>
void dbg(Head H,Tail... T) {
    _print(H);
    if(sizeof...(T)) cerr<<",";
    else cerr<<"\"]";
    dbg(T...);
}
#define debug(...) cerr<<"["<<#__VA_ARGS__<<"]:[\"",dbg(__VA_ARGS__)

int n;
vector<bool> flag;
vector<int> a;
int divv;
bool ok=false;

void gen(int sum,int cnt,vector<bool> &flag) {
//    debug(sum,cnt);
    if(sum<0) return;
    if(ok) return;
    if(sum==0) {
        if(cnt==3) {
            for(int i=1;i<=n;i++) cout<<a[i]<<" "; cout<<endl;
            exit(0);
            ok=true; return;
        }
        else {
            gen(divv+cnt-1,cnt+1,flag);
        }
        return;
    }
    for(int i=1;i<=n;i++) {
        if(flag[i] or sum-i<0) continue;
        flag[i]=true; a[i]=cnt;
        gen(sum-i,cnt,flag);
        flag[i]=false; a[i]=0;
    }
}

int main () {
    ios::sync_with_stdio(false); cin.tie(0);
    cin>>n; flag.resize(n+1,false); a.resize(n+1,0);
    int sum=n*(n+1)/2;
    if(sum%3!=0) cout<<"IMPOSSIBLE"<<endl;
    else {
        divv=sum/3;
        gen(divv-1,1,flag);
//        for(int i=1;i<=n;i++) cout<<a[i]<<" "; cout<<endl;
    }
}

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