Building Teams

#include <bits/stdc++.h> 
using namespace std;
#define REP(i,a,b) for (int i = a; i < b; i++)

int main(){
    int n;
    int m;
    cin>>n>>m;
    bool impossible = false;

    vector<vector<int>> adj;
    REP(i,0,m){
        int a;
        int b;
        cin>>a>>b;
        a--;b--;
        adj[a].push_back(b);
        adj[b].push_back(a);
    }


    queue<int> q;
    bool visited[n];
    int distance[n]; 

    REP(i,0,n){
    if (visited[i]) continue;
    else{
    visited[i] = true;
    distance[i] = 0;
    q.push(i);
    while (!q.empty()) {
        int s = q.front(); q.pop();
        // process node s
        for (auto u : adj[s]) {
            if (visited[u]&&(distance[u]==distance[s])) {impossible= true; continue;};
            if (visited[u]&&(distance[u]!=distance[s])) continue;
            visited[u] = true;
            distance[u] = (distance[s]+1)%2;
            q.push(u);
        }
        }
    }}

    if (impossible){
        cout<<"IMPOSSIBLE";
    }
    else{
    REP(i,0,n){
        cout<<(distance[i]+1);
    }}


    return 0;
}

Test 1

Verdict: RUNTIME ERROR

Test 2

Verdict: RUNTIME ERROR

Test 3

Verdict: RUNTIME ERROR

Test 4

Verdict: RUNTIME ERROR

Test 5

Verdict: RUNTIME ERROR

Test 6

Verdict: RUNTIME ERROR

Test 7

Verdict: RUNTIME ERROR

Test 8

Verdict: RUNTIME ERROR

Test 9

Verdict: RUNTIME ERROR

Test 10

Verdict: RUNTIME ERROR

Test 11

Verdict: RUNTIME ERROR

Test 12

Verdict: RUNTIME ERROR