Problem p02697 (C++) — buggy submission, verdict: Runtime Error

#pragma GCC optimize("Ofast")
#include <algorithm>
#include <bitset>
#include <cassert>
#include <cmath>
#include <complex>
#include <cstdio>
#include <cstdlib>
#include <functional>
#include <iomanip>
#include <iostream>
#include <limits>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <tuple>
#include <vector>

using namespace std;

using ll = long long int;
using pii = pair<int, int>;
using pil = pair<int, ll>;
using pli = pair<ll, int>;
using pll = pair<ll, ll>;
using psi = pair<string, int>;
using pis = pair<int, string>;
using psl = pair<string, ll>;
using pls = pair<ll, string>;
using pss = pair<string, string>;

// 辺 fromあり
template <typename T> struct Edge {
  int from, to;
  T cost;

  Edge() = default;
  Edge(int from, int to, T cost) : from(from), to(to), cost(cost) {}

  inline bool operator<(const Edge p) const noexcept { return cost < p.cost; }

  inline bool operator>(const Edge p) const noexcept { return cost > p.cost; }
};

// 辺 fromがない
template <typename T> struct edge {
  int to;
  T cost;
  edge() = default;
  edge(int to, T cost) : to(to), cost(cost) {}
};
template <typename T> using edges = vector<edge<T>>;
template <typename T> using WeightGraph = vector<edges<T>>;

using Graph = vector<vector<int>>;

template <typename T> using vc = vector<T>;
template <typename T> using vvc = vector<vector<T>>;
template <typename T> using vvvc = vector<vector<vector<T>>>;
template <typename T> using vvvvc = vector<vvvc<T>>;
template <typename T> using vvvvvc = vector<vvvvc<T>>;

template <class T, class U> inline constexpr bool chmin(T &a, const U b) {
  if (a <= b)
    return false;
  a = b;
  return true;
}
template <class T, class U> inline constexpr bool chmax(T &a, const U b) {
  if (a >= b)
    return false;
  a = b;
  return true;
}
#define bit(n, k) ((n >> k) & 1)

inline void bin101() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);
  cout << fixed << setprecision(20);
}

template <typename T> inline void Yes(T flag) {
  if (flag)
    cout << "Yes" << endl;
  else
    cout << "No" << endl;
}

template <typename T> inline void YES(T flag) {
  if (flag)
    cout << "YES" << endl;
  else
    cout << "NO" << endl;
}

// 1-indexed vector cin
template <typename T> inline void vin1(vector<T> &v) {
  for (int i = 1; i < v.size(); i++)
    cin >> v[i];
}

// 0-indexed vector cin
template <typename T> inline void vin0(vector<T> &v) {
  for (int i = 0; i < v.size(); i++)
    cin >> v[i];
}

// 1-indexed vector<vector> cin
template <typename T> inline void vin1(vector<vector<T>> &v) {
  for (int i = 1; i < v.size(); i++) {
    for (int j = 1; j < v[i].size(); j++)
      cin >> v[i][j];
  }
}

// 0-indexed vector<vector> cin
template <typename T> inline void vin0(vector<vector<T>> &v) {
  for (int i = 0; i < v.size(); i++) {
    for (int j = 0; j < v[i].size(); j++)
      cin >> v[i][j];
  }
}

// デバッグ
template <typename T> inline void vout(const vector<T> &v) {
  cout << "\\nstart\\n";
  const int sz = v.size();
  for (int i = 0; i < sz; i++) {
    cout << i << " " << v[i] << '\\n';
  }
  cout << "end\\n" << endl;
}
// デバッグ
template <typename T> inline void vout(const vvc<T> &v) {
  cout << "\\nstart\\n";
  const int sz = v.size();
  for (int i = 0; i < sz; i++) {
    int ssz = v[i].size();
    for (int j = 0; j < ssz; j++) {
      cout << i << " " << j << " " << v[i][j] << '\\n';
    }
  }
  cout << "\\nend\\n" << endl;
}

// デバッグ(グリッド)
template <typename T> inline void gvout(const vector<T> &v) {
  cout << "\\nstart\\n";
  const int sz = v.size();
  for (int i = 0; i < sz; i++) {
    if (i)
      cout << " ";
    cout << v[i];
  }
  cout << "\\nend\\n" << endl;
}
// デバッグ(グリッド)
template <typename T> inline void gvout(const vvc<T> &v) {
  cout << "\\nstart\\n";
  const int sz = v.size();
  for (int i = 0; i < sz; i++) {
    int ssz = v[i].size();
    for (int j = 0; j < ssz; j++) {
      if (j)
        cout << " ";
      cout << v[i][j];
    }
    cout << endl;
  }
  cout << "end\\n" << endl;
}

// デバッグ
template <typename T> inline void vout(const vvvc<T> &v) {
  cout << "\\nstart\\n";
  const int sz = v.size();
  for (int i = 0; i < sz; i++) {
    int ssz = v[i].size();
    for (int j = 0; j < ssz; j++) {
      int sssz = v[i][j].size();
      for (int k = 0; k < sssz; k++) {
        cout << i << " " << j << " " << k << " " << v[i][j][k] << '\\n';
      }
    }
  }
  cout << "end\\n" << endl;
}

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

// pair cin
template <typename T, typename U>
inline istream &operator>>(istream &is, pair<T, U> &p) {
  is >> p.first >> p.second;
  return is;
}

// ソート
template <typename T> inline void vsort(vector<T> &v) {
  sort(v.begin(), v.end());
}

// 逆順ソート
template <typename T> inline void rvsort(vector<T> &v) {
  sort(v.rbegin(), v.rend());
}

// 要素数a 初期値x
template <typename T> inline vector<T> vmake(int a, T x) {
  return vector<T>(a, x);
}

// data[a][b] 初期値x
template <typename T> inline vector<vector<T>> vmake(int a, int b, T x) {
  return vector<vector<T>>(a, vector<T>(b, x));
}

// data[a][b][c] 初期値x
template <typename T>
inline vector<vector<vector<T>>> vmake(int a, int b, int c, T x) {
  return vector<vector<vector<T>>>(a, vector<vector<T>>(b, vector<T>(c, x)));
}

// data[a][b][c][d]　初期値x
template <typename T>
inline vector<vector<vector<vector<T>>>> vmake(int a, int b, int c, int d,
                                               T x) {
  return vector<vector<vector<vector<T>>>>(a,
                                           vvvc<T>(b, vvc<T>(c, vc<T>(d, x))));
}

// data[a][b][c][d][e] 初期値x
template <typename T>
inline vvvvvc<T> vmake(int a, int b, int c, int d, int e, T x) {
  return vvvvvc<T>(a, vvvvc<T>(b, vvvc<T>(c, vvc<T>(d, vc<T>(e, x)))));
}

// 1ビットの数を返す
inline int popcount(int x) { return __builtin_popcount(x); }
// 1ビットの数を返す
inline int popcount(ll x) { return __builtin_popcountll(x); }

// queのfront() pop()
template <typename T> inline T pop(queue<T> &que) {
  assert(!que.empty());
  T x = que.front();
  que.pop();
  return x;
}

// priority_que top() pop()
template <typename T> inline T pop(priority_queue<T> &que) {
  assert(!que.empty());
  T x = que.top();
  que.pop();
  return x;
}

// stack top() pop()
template <typename T> inline T pop(stack<T> &st) {
  assert(!st.empty());
  T x = st.top();
  st.pop();
  return x;
}

#define SZ(x) ((int)x.size())
#define pb push_back
#define eb emplace_back

/*
満たすものの個数を返す
mode:0 x未満
mode:1 x以下の数
mode:2 x以上の数
mode:3 x超
mode:4 x
*/
template <typename T> inline int count_bound(vector<T> &v, T x, int mode) {
  switch (mode) {
  case 0:
    return lower_bound(v.begin(), v.end(), x) - v.begin();
  case 1:
    return upper_bound(v.begin(), v.end(), x) - v.begin();
  case 2:
    return v.end() - lower_bound(v.begin(), v.end(), x);
  case 3:
    return v.end() - upper_bound(v.begin(), v.end(), x);
  case 4:
    return upper_bound(v.begin(), v.end(), x) -
           lower_bound(v.begin(), v.end(), x);
  }
}

/*
mode:0 xより小さい数で最大の数
mode:1 x以下の数で最大の数
mode:2 x以上の数で最小の数
mode:3 xより大きい数で最小の数
*/
template <typename T> inline T value_bound(vector<T> &v, T x, int mode) {
  switch (mode) {
  case 0:
    return *(--lower_bound(v.begin(), v.end(), x));
  case 1:
    return *(--upper_bound(v.begin(), v.end(), x));
  case 2:
    return *lower_bound(v.begin(), v.end(), x);
  case 3:
    return *upper_bound(v.begin(), v.end(), x);
  }
}

constexpr int MAX = 1 << 30;
constexpr ll INF = 1LL << 62;
constexpr ll MOD = 1e9 + 7;
constexpr int dx[] = {1, -1, 0, 0}, dy[] = {0, 0, -1, 1};

//+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
//+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
//+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

int main() {
  bin101();

  int N, M;
  cin >> N >> M;

  vector<int> used(N + 1);
  int s = 1;
  int m = M;
  if (m % 2 == 1)
    ;
  else
    m--;
  for (int i = m; i >= 1; i -= 2) {
    cout << s << " " << s + i << endl;
    s++;
  }
  s = 1 + m + 1;
  m = M;
  if (m % 2 == 0)
    ;
  else
    m--;
  for (int i = m; i >= 1; i -= 2) {
    cout << s << " " << s + i << endl;
    s++;
  }
}


Error: WA

Fixed submission:

#pragma GCC optimize("Ofast")
#include <algorithm>
#include <bitset>
#include <cassert>
#include <cmath>
#include <complex>
#include <cstdio>
#include <cstdlib>
#include <functional>
#include <iomanip>
#include <iostream>
#include <limits>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <tuple>
#include <vector>

using namespace std;

using ll = long long int;
using pii = pair<int, int>;
using pil = pair<int, ll>;
using pli = pair<ll, int>;
using pll = pair<ll, ll>;
using psi = pair<string, int>;
using pis = pair<int, string>;
using psl = pair<string, ll>;
using pls = pair<ll, string>;
using pss = pair<string, string>;

// 辺 fromあり
template <typename T> struct Edge {
  int from, to;
  T cost;

  Edge() = default;
  Edge(int from, int to, T cost) : from(from), to(to), cost(cost) {}

  inline bool operator<(const Edge p) const noexcept { return cost < p.cost; }

  inline bool operator>(const Edge p) const noexcept { return cost > p.cost; }
};

// 辺 fromがない
template <typename T> struct edge {
  int to;
  T cost;
  edge() = default;
  edge(int to, T cost) : to(to), cost(cost) {}
};
template <typename T> using edges = vector<edge<T>>;
template <typename T> using WeightGraph = vector<edges<T>>;

using Graph = vector<vector<int>>;

template <typename T> using vc = vector<T>;
template <typename T> using vvc = vector<vector<T>>;
template <typename T> using vvvc = vector<vector<vector<T>>>;
template <typename T> using vvvvc = vector<vvvc<T>>;
template <typename T> using vvvvvc = vector<vvvvc<T>>;

template <class T, class U> inline constexpr bool chmin(T &a, const U b) {
  if (a <= b)
    return false;
  a = b;
  return true;
}
template <class T, class U> inline constexpr bool chmax(T &a, const U b) {
  if (a >= b)
    return false;
  a = b;
  return true;
}
#define bit(n, k) ((n >> k) & 1)

inline void bin101() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);
  cout << fixed << setprecision(20);
}

template <typename T> inline void Yes(T flag) {
  if (flag)
    cout << "Yes" << endl;
  else
    cout << "No" << endl;
}

template <typename T> inline void YES(T flag) {
  if (flag)
    cout << "YES" << endl;
  else
    cout << "NO" << endl;
}

// 1-indexed vector cin
template <typename T> inline void vin1(vector<T> &v) {
  for (int i = 1; i < v.size(); i++)
    cin >> v[i];
}

// 0-indexed vector cin
template <typename T> inline void vin0(vector<T> &v) {
  for (int i = 0; i < v.size(); i++)
    cin >> v[i];
}

// 1-indexed vector<vector> cin
template <typename T> inline void vin1(vector<vector<T>> &v) {
  for (int i = 1; i < v.size(); i++) {
    for (int j = 1; j < v[i].size(); j++)
      cin >> v[i][j];
  }
}

// 0-indexed vector<vector> cin
template <typename T> inline void vin0(vector<vector<T>> &v) {
  for (int i = 0; i < v.size(); i++) {
    for (int j = 0; j < v[i].size(); j++)
      cin >> v[i][j];
  }
}

// デバッグ
template <typename T> inline void vout(const vector<T> &v) {
  cout << "\\nstart\\n";
  const int sz = v.size();
  for (int i = 0; i < sz; i++) {
    cout << i << " " << v[i] << '\\n';
  }
  cout << "end\\n" << endl;
}
// デバッグ
template <typename T> inline void vout(const vvc<T> &v) {
  cout << "\\nstart\\n";
  const int sz = v.size();
  for (int i = 0; i < sz; i++) {
    int ssz = v[i].size();
    for (int j = 0; j < ssz; j++) {
      cout << i << " " << j << " " << v[i][j] << '\\n';
    }
  }
  cout << "\\nend\\n" << endl;
}

// デバッグ(グリッド)
template <typename T> inline void gvout(const vector<T> &v) {
  cout << "\\nstart\\n";
  const int sz = v.size();
  for (int i = 0; i < sz; i++) {
    if (i)
      cout << " ";
    cout << v[i];
  }
  cout << "\\nend\\n" << endl;
}
// デバッグ(グリッド)
template <typename T> inline void gvout(const vvc<T> &v) {
  cout << "\\nstart\\n";
  const int sz = v.size();
  for (int i = 0; i < sz; i++) {
    int ssz = v[i].size();
    for (int j = 0; j < ssz; j++) {
      if (j)
        cout << " ";
      cout << v[i][j];
    }
    cout << endl;
  }
  cout << "end\\n" << endl;
}

// デバッグ
template <typename T> inline void vout(const vvvc<T> &v) {
  cout << "\\nstart\\n";
  const int sz = v.size();
  for (int i = 0; i < sz; i++) {
    int ssz = v[i].size();
    for (int j = 0; j < ssz; j++) {
      int sssz = v[i][j].size();
      for (int k = 0; k < sssz; k++) {
        cout << i << " " << j << " " << k << " " << v[i][j][k] << '\\n';
      }
    }
  }
  cout << "end\\n" << endl;
}

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

// pair cin
template <typename T, typename U>
inline istream &operator>>(istream &is, pair<T, U> &p) {
  is >> p.first >> p.second;
  return is;
}

// ソート
template <typename T> inline void vsort(vector<T> &v) {
  sort(v.begin(), v.end());
}

// 逆順ソート
template <typename T> inline void rvsort(vector<T> &v) {
  sort(v.rbegin(), v.rend());
}

// 要素数a 初期値x
template <typename T> inline vector<T> vmake(int a, T x) {
  return vector<T>(a, x);
}

// data[a][b] 初期値x
template <typename T> inline vector<vector<T>> vmake(int a, int b, T x) {
  return vector<vector<T>>(a, vector<T>(b, x));
}

// data[a][b][c] 初期値x
template <typename T>
inline vector<vector<vector<T>>> vmake(int a, int b, int c, T x) {
  return vector<vector<vector<T>>>(a, vector<vector<T>>(b, vector<T>(c, x)));
}

// data[a][b][c][d]　初期値x
template <typename T>
inline vector<vector<vector<vector<T>>>> vmake(int a, int b, int c, int d,
                                               T x) {
  return vector<vector<vector<vector<T>>>>(a,
                                           vvvc<T>(b, vvc<T>(c, vc<T>(d, x))));
}

// data[a][b][c][d][e] 初期値x
template <typename T>
inline vvvvvc<T> vmake(int a, int b, int c, int d, int e, T x) {
  return vvvvvc<T>(a, vvvvc<T>(b, vvvc<T>(c, vvc<T>(d, vc<T>(e, x)))));
}

// 1ビットの数を返す
inline int popcount(int x) { return __builtin_popcount(x); }
// 1ビットの数を返す
inline int popcount(ll x) { return __builtin_popcountll(x); }

// queのfront() pop()
template <typename T> inline T pop(queue<T> &que) {
  assert(!que.empty());
  T x = que.front();
  que.pop();
  return x;
}

// priority_que top() pop()
template <typename T> inline T pop(priority_queue<T> &que) {
  assert(!que.empty());
  T x = que.top();
  que.pop();
  return x;
}

// stack top() pop()
template <typename T> inline T pop(stack<T> &st) {
  assert(!st.empty());
  T x = st.top();
  st.pop();
  return x;
}

#define SZ(x) ((int)x.size())
#define pb push_back
#define eb emplace_back

/*
満たすものの個数を返す
mode:0 x未満
mode:1 x以下の数
mode:2 x以上の数
mode:3 x超
mode:4 x
*/
template <typename T> inline int count_bound(vector<T> &v, T x, int mode) {
  switch (mode) {
  case 0:
    return lower_bound(v.begin(), v.end(), x) - v.begin();
  case 1:
    return upper_bound(v.begin(), v.end(), x) - v.begin();
  case 2:
    return v.end() - lower_bound(v.begin(), v.end(), x);
  case 3:
    return v.end() - upper_bound(v.begin(), v.end(), x);
  case 4:
    return upper_bound(v.begin(), v.end(), x) -
           lower_bound(v.begin(), v.end(), x);
  }
}

/*
mode:0 xより小さい数で最大の数
mode:1 x以下の数で最大の数
mode:2 x以上の数で最小の数
mode:3 xより大きい数で最小の数
*/
template <typename T> inline T value_bound(vector<T> &v, T x, int mode) {
  switch (mode) {
  case 0:
    return *(--lower_bound(v.begin(), v.end(), x));
  case 1:
    return *(--upper_bound(v.begin(), v.end(), x));
  case 2:
    return *lower_bound(v.begin(), v.end(), x);
  case 3:
    return *upper_bound(v.begin(), v.end(), x);
  }
}

constexpr int MAX = 1 << 30;
constexpr ll INF = 1LL << 62;
constexpr ll MOD = 1e9 + 7;
constexpr int dx[] = {1, -1, 0, 0}, dy[] = {0, 0, -1, 1};

//+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
//+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
//+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

int main() {
  bin101();

  int N, M;
  cin >> N >> M;

  vector<int> used(N + 1);
  int s = 1;
  for (int i = 1; i <= M; i++) {
    while (used[s])
      s++;
    cout << s << " " << s + i << endl;
    used[s + i] = true;
    s++;
  }
}