Problem p03647 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
using namespace std;
#define F first
#define S second
#define R cin >>
#define Z class
#define ll long long
#define ln cout << '\n'
#define in(a) insert(a)
#define pb(a) push_back(a)
#define pd(a) printf("%.10f\n", a)
#define mem(a) memset(a, 0, sizeof(a))
#define all(c) (c).begin(), (c).end()
#define iter(c) __typeof((c).begin())
#define rrep(i, n) for (int i = (int)(n)-1; i >= 0; i--)
#define REP(i, m, n) for (int i = (int)(m); i < (int)(n); i++)
#define rep(i, n) REP(i, 0, n)
#define tr(it, c) for (iter(c) it = (c).begin(); it != (c).end(); it++)
template <Z A> void pr(A a) {
  cout << a;
  ln;
}
template <Z A, Z B> void pr(A a, B b) {
  cout << a << ' ';
  pr(b);
}
template <Z A, Z B, Z C> void pr(A a, B b, C c) {
  cout << a << ' ';
  pr(b, c);
}
template <Z A, Z B, Z C, Z D> void pr(A a, B b, C c, D d) {
  cout << a << ' ';
  pr(b, c, d);
}
template <Z A> void PR(A a, ll n) {
  rep(i, n) {
    if (i)
      cout << ' ';
    cout << a[i];
  }
  ln;
}
ll check(ll n, ll m, ll x, ll y) { return x >= 0 && x < n && y >= 0 && y < m; }
const ll MAX = 1000000007, MAXL = 1LL << 61, dx[4] = {-1, 0, 1, 0},
         dy[4] = {0, 1, 0, -1};
typedef pair<int, int> P;

void Main() {
  int n, m;
  cin >> n >> m;
  vector<int> v[n];
  rep(i, m) {
    int x, y;
    cin >> x >> y;
    x--, y--;
    v[x].pb(y);
    v[y].pb(x);
  }
  bitset<100000> s, t;
  s.set(0);
  t.set(n - 1);
  rep(i, v[0].size()) s.set(v[0][i]);
  rep(i, v[n - 1].size()) t.set(v[n - 1][i]);
  if ((s & t).count())
    pr("POSSIBLE");
  else
    pr("IMPOSSIBLE");
}

int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  Main();
  return 0;
}


Fixed submission:
#include <bits/stdc++.h>
using namespace std;
#define F first
#define S second
#define R cin >>
#define Z class
#define ll long long
#define ln cout << '\n'
#define in(a) insert(a)
#define pb(a) push_back(a)
#define pd(a) printf("%.10f\n", a)
#define mem(a) memset(a, 0, sizeof(a))
#define all(c) (c).begin(), (c).end()
#define iter(c) __typeof((c).begin())
#define rrep(i, n) for (int i = (int)(n)-1; i >= 0; i--)
#define REP(i, m, n) for (int i = (int)(m); i < (int)(n); i++)
#define rep(i, n) REP(i, 0, n)
#define tr(it, c) for (iter(c) it = (c).begin(); it != (c).end(); it++)
template <Z A> void pr(A a) {
  cout << a;
  ln;
}
template <Z A, Z B> void pr(A a, B b) {
  cout << a << ' ';
  pr(b);
}
template <Z A, Z B, Z C> void pr(A a, B b, C c) {
  cout << a << ' ';
  pr(b, c);
}
template <Z A, Z B, Z C, Z D> void pr(A a, B b, C c, D d) {
  cout << a << ' ';
  pr(b, c, d);
}
template <Z A> void PR(A a, ll n) {
  rep(i, n) {
    if (i)
      cout << ' ';
    cout << a[i];
  }
  ln;
}
ll check(ll n, ll m, ll x, ll y) { return x >= 0 && x < n && y >= 0 && y < m; }
const ll MAX = 1000000007, MAXL = 1LL << 61, dx[4] = {-1, 0, 1, 0},
         dy[4] = {0, 1, 0, -1};
typedef pair<int, int> P;

void Main() {
  int n, m;
  cin >> n >> m;
  vector<int> v[n];
  rep(i, m) {
    int x, y;
    cin >> x >> y;
    x--, y--;
    v[x].pb(y);
    v[y].pb(x);
  }
  bitset<200000> s, t;
  s.set(0);
  t.set(n - 1);
  rep(i, v[0].size()) s.set(v[0][i]);
  rep(i, v[n - 1].size()) t.set(v[n - 1][i]);
  if ((s & t).count())
    pr("POSSIBLE");
  else
    pr("IMPOSSIBLE");
}

int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  Main();
  return 0;
}