Problem p03203 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
using namespace std;

template <class T> bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> bool chmin(T &a, const T &b) {
  if (b < a) {
    a = b;
    return true;
  }
  return false;
}

// std::vector Declaration
template <typename T> vector<T> make_v(size_t a) { return vector<T>(a); }
template <typename T, typename... Ts> auto make_v(size_t a, Ts... ts) {
  return vector<decltype(make_v<T>(ts...))>(a, make_v<T>(ts...));
}

// std::vector Declaration and Initialization
template <typename T> vector<T> make_vector(size_t a, T x) {
  return vector<T>(a, x);
}
template <typename T, typename U, typename... Ts>
auto make_vector(size_t a, U b, Ts... ts) {
  return vector<decltype(make_vector<T>(b, ts...))>(a,
                                                    make_vector<T>(b, ts...));
}

// std::vector Input
template <typename T> istream &operator>>(istream &is, vector<T> &v) {
  for (auto &e : v)
    is >> e;
  return is;
}

// std::vector Debug
template <typename T> ostream &operator<<(ostream &os, const vector<T> &v) {
  os << "[";
  bool a = 1;
  for (auto e : v) {
    os << (a ? "" : " ");
    os << e;
    a = 0;
  }
  os << "]";
  return os;
}

// std::array Debug
template <typename T, size_t n>
ostream &operator<<(ostream &os, const array<T, n> &v) {
  os << "[";
  bool a = 1;
  for (auto e : v) {
    os << (a ? "" : " ");
    os << e;
    a = 0;
  }
  os << "]";
  return os;
}

// std::deque Debug
template <typename T> ostream &operator<<(ostream &os, const deque<T> &d) {
  os << "[";
  bool a = 1;
  for (auto e : d) {
    os << (a ? "" : " ");
    os << e;
    a = 0;
  }
  os << "]";
  return os;
}

// std::pair Debug
template <typename T, typename U>
ostream &operator<<(ostream &os, const pair<T, U> &p) {
  os << "(" << p.first << " " << p.second << ")";
  return os;
}

// std::set Debug
template <typename T> ostream &operator<<(ostream &os, const set<T> &st) {
  os << "{";
  bool a = 1;
  for (auto e : st) {
    os << (a ? "" : " ");
    os << e;
    a = 0;
  }
  os << "}";
  return os;
}

// std::multiset Debug
template <typename T> ostream &operator<<(ostream &os, const multiset<T> &st) {
  os << "{";
  bool a = 1;
  for (auto e : st) {
    os << (a ? "" : " ");
    os << e;
    a = 0;
  }
  os << "}";
  return os;
}

// std::map Debug
template <typename T, typename U>
ostream &operator<<(ostream &os, const map<T, U> &mp) {
  os << "{";
  bool a = 1;
  for (auto e : mp) {
    os << (a ? "" : " ");
    os << e.first << ":" << e.second;
    a = 0;
  }
  os << "}";
  return os;
}

// std::tuple Debug
template <int N, class Tuple> void out(ostream &os, const Tuple &t) {}
template <int N, class Tuple, class H, class... Ts>
void out(ostream &os, const Tuple &t) {
  if (N)
    os << " ";
  os << get<N>(t);
  out<N + 1, Tuple, Ts...>(os, t);
}
template <class... Ts> ostream &operator<<(ostream &os, const tuple<Ts...> &t) {
  os << "(";
  out<0, tuple<Ts...>, Ts...>(os, t);
  os << ")";
  return os;
}

// Debug
#define DUMP(x) cerr << #x << " = " << (x) << endl

// Weighted edge
template <typename T> struct edge {
  int src, to;
  T cost;

  edge(int to, T cost) : src(-1), to(to), cost(cost) {}
  edge(int src, int to, T cost) : src(src), to(to), cost(cost) {}

  friend ostream &operator<<(ostream &os, const edge &e) {
    return os << "(" << e.src << "->" << e.to << ":" << e.cost << ")";
  }
};

using LL = long long;

#define fs first
#define sc second

const LL MOD = 1e9 + 7;

int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  cout << fixed << setprecision(10);

  int H, W, N;
  cin >> H >> W >> N;
  map<pair<int, int>, int> object;
  vector<vector<int>> objectOnCol(W);
  for (int i = 0; i < N; ++i) {
    int x, y;
    cin >> x >> y;
    --x, --y;
    object[make_pair(x, y)];
    objectOnCol[y].emplace_back(x);
  }

  for (int y = 0; y < W; ++y) {
    sort(objectOnCol[y].begin(), objectOnCol[y].end());
    objectOnCol[y].emplace_back(H);
  }

  // DUMP(objectOnCol);

  vector<int> reachableHeightOnCol(W, -1);
  reachableHeightOnCol[0] = 0;
  int x = 0, y = 0;
  while (x < H) {
    if (object.count({x + 1, y}) == 0) {
      ++x;
    } else {
      break;
    }
    if (object.count({x, y + 1}) == 0) {
      ++y;
      if (y >= H) {
        break;
      } else {
        reachableHeightOnCol[y] = x;
      }
    }
  }
  // DUMP(reachableHeightOnCol);

  int ans = H + 1;
  for (int y = 0; y < W; ++y) {
    if (reachableHeightOnCol[y] == -1) {
      break;
    }
    int r = reachableHeightOnCol[y];
    auto lb = lower_bound(objectOnCol[y].begin(), objectOnCol[y].end(), r);
    chmin(ans, *lb);
  }

  cout << ans << endl;

  return 0;
}


Fixed submission:
#include <bits/stdc++.h>
using namespace std;

template <class T> bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> bool chmin(T &a, const T &b) {
  if (b < a) {
    a = b;
    return true;
  }
  return false;
}

// std::vector Declaration
template <typename T> vector<T> make_v(size_t a) { return vector<T>(a); }
template <typename T, typename... Ts> auto make_v(size_t a, Ts... ts) {
  return vector<decltype(make_v<T>(ts...))>(a, make_v<T>(ts...));
}

// std::vector Declaration and Initialization
template <typename T> vector<T> make_vector(size_t a, T x) {
  return vector<T>(a, x);
}
template <typename T, typename U, typename... Ts>
auto make_vector(size_t a, U b, Ts... ts) {
  return vector<decltype(make_vector<T>(b, ts...))>(a,
                                                    make_vector<T>(b, ts...));
}

// std::vector Input
template <typename T> istream &operator>>(istream &is, vector<T> &v) {
  for (auto &e : v)
    is >> e;
  return is;
}

// std::vector Debug
template <typename T> ostream &operator<<(ostream &os, const vector<T> &v) {
  os << "[";
  bool a = 1;
  for (auto e : v) {
    os << (a ? "" : " ");
    os << e;
    a = 0;
  }
  os << "]";
  return os;
}

// std::array Debug
template <typename T, size_t n>
ostream &operator<<(ostream &os, const array<T, n> &v) {
  os << "[";
  bool a = 1;
  for (auto e : v) {
    os << (a ? "" : " ");
    os << e;
    a = 0;
  }
  os << "]";
  return os;
}

// std::deque Debug
template <typename T> ostream &operator<<(ostream &os, const deque<T> &d) {
  os << "[";
  bool a = 1;
  for (auto e : d) {
    os << (a ? "" : " ");
    os << e;
    a = 0;
  }
  os << "]";
  return os;
}

// std::pair Debug
template <typename T, typename U>
ostream &operator<<(ostream &os, const pair<T, U> &p) {
  os << "(" << p.first << " " << p.second << ")";
  return os;
}

// std::set Debug
template <typename T> ostream &operator<<(ostream &os, const set<T> &st) {
  os << "{";
  bool a = 1;
  for (auto e : st) {
    os << (a ? "" : " ");
    os << e;
    a = 0;
  }
  os << "}";
  return os;
}

// std::multiset Debug
template <typename T> ostream &operator<<(ostream &os, const multiset<T> &st) {
  os << "{";
  bool a = 1;
  for (auto e : st) {
    os << (a ? "" : " ");
    os << e;
    a = 0;
  }
  os << "}";
  return os;
}

// std::map Debug
template <typename T, typename U>
ostream &operator<<(ostream &os, const map<T, U> &mp) {
  os << "{";
  bool a = 1;
  for (auto e : mp) {
    os << (a ? "" : " ");
    os << e.first << ":" << e.second;
    a = 0;
  }
  os << "}";
  return os;
}

// std::tuple Debug
template <int N, class Tuple> void out(ostream &os, const Tuple &t) {}
template <int N, class Tuple, class H, class... Ts>
void out(ostream &os, const Tuple &t) {
  if (N)
    os << " ";
  os << get<N>(t);
  out<N + 1, Tuple, Ts...>(os, t);
}
template <class... Ts> ostream &operator<<(ostream &os, const tuple<Ts...> &t) {
  os << "(";
  out<0, tuple<Ts...>, Ts...>(os, t);
  os << ")";
  return os;
}

// Debug
#define DUMP(x) cerr << #x << " = " << (x) << endl

// Weighted edge
template <typename T> struct edge {
  int src, to;
  T cost;

  edge(int to, T cost) : src(-1), to(to), cost(cost) {}
  edge(int src, int to, T cost) : src(src), to(to), cost(cost) {}

  friend ostream &operator<<(ostream &os, const edge &e) {
    return os << "(" << e.src << "->" << e.to << ":" << e.cost << ")";
  }
};

using LL = long long;

#define fs first
#define sc second

const LL MOD = 1e9 + 7;

int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  cout << fixed << setprecision(10);

  int H, W, N;
  cin >> H >> W >> N;
  map<pair<int, int>, int> object;
  vector<vector<int>> objectOnCol(W);
  for (int i = 0; i < N; ++i) {
    int x, y;
    cin >> x >> y;
    --x, --y;
    object[make_pair(x, y)];
    objectOnCol[y].emplace_back(x);
  }

  for (int y = 0; y < W; ++y) {
    sort(objectOnCol[y].begin(), objectOnCol[y].end());
    objectOnCol[y].emplace_back(H);
  }

  // DUMP(objectOnCol);

  vector<int> reachableHeightOnCol(W, -1);
  reachableHeightOnCol[0] = 0;
  int x = 0, y = 0;
  while (x < H) {
    if (object.count({x + 1, y}) == 0) {
      ++x;
    } else {
      break;
    }
    if (object.count({x, y + 1}) == 0) {
      ++y;
      if (y >= W) {
        break;
      } else {
        reachableHeightOnCol[y] = x;
      }
    }
  }
  // DUMP(reachableHeightOnCol);

  int ans = H + 1;
  for (int y = 0; y < W; ++y) {
    if (reachableHeightOnCol[y] == -1) {
      break;
    }
    int r = reachableHeightOnCol[y];
    auto lb = lower_bound(objectOnCol[y].begin(), objectOnCol[y].end(), r);
    chmin(ans, *lb);
  }

  cout << ans << endl;

  return 0;
}