Problem p03647 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <algorithm>
#include <bitset>
#include <complex>
#include <functional>
#include <iomanip>
#include <iostream>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <utility>
#include <valarray>
#include <vector>

#include <cctype>
#include <cmath>
#include <cstdio>
#include <cstdlib>
#include <cstring>

using namespace std;

#define all(c) c.begin(), c.end()
#define repeat(i, n) for (int i = 0; i < static_cast<int>(n); i++)
#define debug(x) #x << "=" << (x)
#define dump(x) cerr << debug(x) << " (L:" << __LINE__ << ")" << endl

typedef long long ll;
typedef vector<int> vi;
typedef vector<vector<int>> vvi;
typedef vector<long> vl;
typedef vector<vector<long>> vvl;
typedef vector<string> vs;

template <typename T> ostream &operator<<(ostream &os, const vector<T> &vec) {
  os << "[";
  for (int i = 0; i < vec.size(); i++) {
    os << vec[i] << ",";
  }
  os << "]";
  return os;
}

template <typename T> T input() {
  T t;
  cin >> t;
  return t;
}

template <typename T> vector<T> input(const int N) {
  vector<T> v(N);
  repeat(i, N) cin >> v[i];
  return v;
}

long long gcd(long long a, long long b) {
  if (b == 0) {
    return a;
  }
  return gcd(b, a % b);
}

long long lcm(long long a, long long b) { return (a / gcd(a, b)) * b; }

long long mul(const long long &a, const long long &b, const long long &mod) {
  return ((a % mod) * (b % mod)) % mod;
}

long long power(const long long &x, const long long &y, const long long &mod) {
  if (y == 0) {
    return 1;
  } else if (y == 1) {
    return x % mod;
  } else {
    long long value = power(x, y / 2, mod);
    if (y % 2 == 0) {
      return mul(value, value, mod);
    } else {
      return mul(value, value, mod) * x % mod;
    }
  }
}

long long div(const long long &a, const long long &b, const long long &mod) {
  return mul(a, power(b, mod - 2, mod), mod);
}

map<long long, long long> factorials;
long long factorial(const long long &n, const long long &mod) {
  if (n == 0 || n == 1) {
    return 1;
  }
  if (factorials[n] != 0) {
    return factorials[n];
  }
  factorials[n] = n * factorial(n - 1, mod) % mod;
  return factorials[n] % mod;
}

long long combination(const long long &n, const long long &x,
                      const long long &mod) {
  long long numerator = 1;
  long long denominator = 1;
  repeat(i, x) {
    numerator *= (n - i) % mod;
    numerator %= mod;
    denominator *= (i + 1) % mod;
    denominator %= mod;
  }
  return div(numerator, denominator, mod);
}

int main() {
  map<int, map<int, int>> connections;
  long N, M;
  cin >> N >> M;

  long a, b;
  repeat(i, M) {
    cin >> a >> b;

    //        cout << "a: " << a << " b: " << b << endl;
    connections[a - 1][b - 1] = 1;
    connections[b - 1][a - 1] = 1;
  }

  repeat(i, connections[0].size()) {
    repeat(j, connections[N - 1].size()) {
      if ((connections[0][i] == 1) && (connections[N - 1][j] == 1) &&
          (i == j)) {
        cout << "POSSIBLE" << endl;
        return 0;
      }
    }
  }
  cout << "IMPOSSIBLE" << endl;
  return 0;
}


Fixed submission:
#include <algorithm>
#include <bitset>
#include <complex>
#include <functional>
#include <iomanip>
#include <iostream>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <utility>
#include <valarray>
#include <vector>

#include <cctype>
#include <cmath>
#include <cstdio>
#include <cstdlib>
#include <cstring>

using namespace std;

#define all(c) c.begin(), c.end()
#define repeat(i, n) for (int i = 0; i < static_cast<int>(n); i++)
#define debug(x) #x << "=" << (x)
#define dump(x) cerr << debug(x) << " (L:" << __LINE__ << ")" << endl

typedef long long ll;
typedef vector<int> vi;
typedef vector<vector<int>> vvi;
typedef vector<long> vl;
typedef vector<vector<long>> vvl;
typedef vector<string> vs;

template <typename T> ostream &operator<<(ostream &os, const vector<T> &vec) {
  os << "[";
  for (int i = 0; i < vec.size(); i++) {
    os << vec[i] << ",";
  }
  os << "]";
  return os;
}

template <typename T> T input() {
  T t;
  cin >> t;
  return t;
}

template <typename T> vector<T> input(const int N) {
  vector<T> v(N);
  repeat(i, N) cin >> v[i];
  return v;
}

long long gcd(long long a, long long b) {
  if (b == 0) {
    return a;
  }
  return gcd(b, a % b);
}

long long lcm(long long a, long long b) { return (a / gcd(a, b)) * b; }

long long mul(const long long &a, const long long &b, const long long &mod) {
  return ((a % mod) * (b % mod)) % mod;
}

long long power(const long long &x, const long long &y, const long long &mod) {
  if (y == 0) {
    return 1;
  } else if (y == 1) {
    return x % mod;
  } else {
    long long value = power(x, y / 2, mod);
    if (y % 2 == 0) {
      return mul(value, value, mod);
    } else {
      return mul(value, value, mod) * x % mod;
    }
  }
}

long long div(const long long &a, const long long &b, const long long &mod) {
  return mul(a, power(b, mod - 2, mod), mod);
}

map<long long, long long> factorials;
long long factorial(const long long &n, const long long &mod) {
  if (n == 0 || n == 1) {
    return 1;
  }
  if (factorials[n] != 0) {
    return factorials[n];
  }
  factorials[n] = n * factorial(n - 1, mod) % mod;
  return factorials[n] % mod;
}

long long combination(const long long &n, const long long &x,
                      const long long &mod) {
  long long numerator = 1;
  long long denominator = 1;
  repeat(i, x) {
    numerator *= (n - i) % mod;
    numerator %= mod;
    denominator *= (i + 1) % mod;
    denominator %= mod;
  }
  return div(numerator, denominator, mod);
}

int main() {
  map<int, map<int, int>> connections;
  long N, M;
  cin >> N >> M;

  long a, b;
  repeat(i, M) {
    cin >> a >> b;

    //        cout << "a: " << a << " b: " << b << endl;
    connections[a - 1][b - 1] = 1;
    connections[b - 1][a - 1] = 1;
  }

  repeat(i, N) {
    if ((connections[0][i] == 1) && (connections[N - 1][i] == 1)) {
      cout << "POSSIBLE" << endl;
      return 0;
    }
  }
  cout << "IMPOSSIBLE" << endl;
  return 0;
}