Building Teams

#include <bits/stdc++.h>

using namespace std;

//Definitions for quicker writing
#define REP(i,a,b) for (int i = a; i < b; i++)

//Typedefs for quicker writing
typedef long long ll;
typedef vector<int> vi;
typedef pair<int,int> pi;

void dfs(int s, int teams[], vi friends[], int prev, bool imp[]){
	if((teams[s]==1 && prev==2) || (teams[s]==2 && prev==1)) return;
	if((teams[s]==2 && prev==2) || (teams[s]==1 && prev==1)){
		imp[0]=true;
		return;
	}
	if(prev == 1){
	       	teams[s] = 2; 
		prev = 2;
	}
	else if (prev == 2){
	       	teams[s] = 1; 
		prev = 1;
	}
	for (auto u: friends[s]){
		dfs(u, teams, friends, prev, imp);
	}
}

int main() {
	//IO optimization
	ios::sync_with_stdio(0);
	cin.tie(0);
	
	//Input definition
	int n, m;
	
	//Read In
	cin >> n >> m;

	vi friends[n];

	for(int i = 0; i < m; i++) {
		int a,b;
		cin >> a >> b;
		a--;
		b--;
		friends[a].push_back(b);
		friends[b].push_back(a);
	}


	//Main part
	int teams[n] = {0};
	bool imp[1] ={false};

	REP(i,0,n){
		sort(friends[i].begin(), friends[i].end());
	}

	dfs(0, teams, friends, 2, imp);

	REP(i,0,n){
		if(teams[i]==0){
			dfs(i, teams, friends, 2, imp);
		}
	}

	//Write out
	if(!imp[0]){
		REP(i,0,n){
			cout << teams[i] << "\n";
		}
	}
	else{
		cout << "IMPOSSIBLE" << "\n";
	}
		
	//Return
	return 0;

}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED

Test 4

Verdict: ACCEPTED

Test 5

Verdict: ACCEPTED

Test 6

Verdict: ACCEPTED

Test 7

Verdict: ACCEPTED

Test 8

Verdict: ACCEPTED

Test 9

Verdict: ACCEPTED

Test 10

Verdict: ACCEPTED

Test 11

Verdict: ACCEPTED

Test 12

Verdict: ACCEPTED