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

#include <bits/stdc++.h>
using namespace std;
typedef long long lint;
typedef vector<lint> vlint;

#define inf 1e+9
#define endl "\\n"
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define rep1(i, n) for (int i = 1; i < (int)(n); i++)
#define rep_rev(i, n) for (int i = n - 1; i >= 0; --i)
#define all(x) (x).begin(), (x).end()

#define pb push_back
#define eb emplace_back
#define Size(c) (int)(c).size()

#define lb(c, x) distance((c).begin(), lower_bound(all(c), (x)))
#define ub(c, x) distance((c).begin(), upper_bound(all(c), (x)))

template <class T> using pq = priority_queue<T>;
template <class T> using pqg = priority_queue<T, vector<T>, greater<T>>;

template <class T, class S> inline bool chmax(T &a, S b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T, class S> inline bool chmin(T &a, S b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> void line(vector<T> &x) {
  for (auto &&xi : x)
    cin >> xi;
}

inline void IN(void) { return; }

template <typename First, typename... Rest>
void IN(First &first, Rest &...rest) {
  cin >> first;
  IN(rest...);
  return;
}

#define INT(...)                                                               \\
  int __VA_ARGS__;                                                             \\
  IN(__VA_ARGS__)
#define LINT(...)                                                              \\
  lint __VA_ARGS__;                                                            \\
  IN(__VA_ARGS__)
#define STR(...)                                                               \\
  string __VA_ARGS__;                                                          \\
  IN(__VA_ARGS__)

template <class T> void UNIQUE(vector<T> &x) {
  sort(all(x));
  x.erase(unique(all(x)), x.end());
}
template <class T> void print_vec(T first, T end) {
  for (auto i = first; i < end - 1; i++)
    cout << *i << " ";
  cout << *(end - 1) << endl;
}

template <class... T> void debug_print(T... args) {
  vector<lint> tmp = initializer_list<lint>{args...};
  print_vec(all(tmp));
}
template <class T> void print(T a) { cout << a << endl; }

vector<string> Yes = {"No", "Yes"};
vector<string> YES = {"NO", "YES"};

int ord(char x) { return (int)(x - 'a'); }
char chr(lint x) { return (char)(x + (lint)('a')); }
lint mod = 1e9 + 7;
// lint mod =

lint sum(vlint a) {
  lint ret = 0;
  for (auto &&v : a)
    ret += v;
  return ret;
}
lint vmini(vlint a, lint &index) {
  lint ret = LLONG_MAX;
  rep(i, Size(a)) {
    if (chmin(ret, a[i]))
      index = i;
  }
  return ret;
}
lint vmaxi(vlint a, lint &index) {
  lint ret = -LLONG_MAX;
  rep(i, Size(a)) {
    if (chmax(ret, a[i]))
      index = i;
  }
  return ret;
}

lint vmin(vlint a) {
  lint ret = LLONG_MAX;
  for (auto &&v : a)
    chmin(ret, v);
  return ret;
}
lint vmax(vlint a) {
  lint ret = -LLONG_MAX;
  for (auto &&v : a)
    chmax(ret, v);
  return ret;
}

int main() {
  INT(n, m);
  vector<vlint> ans;
  vlint check(n + 1, 0);
  int cnt = 1;
  int diff = n - 1;
  while (cnt < cnt + diff) {
    ans.pb({cnt, cnt + diff});
    cnt++;
    diff -= 2;
    if (n % 2 == 0 && diff + 2 > n / 2 && diff <= n / 2)
      diff--;
  }
  rep(i, m) { print_vec(all(ans[i])); }
}


Error: WA

Fixed submission:

#include <bits/stdc++.h>
using namespace std;
typedef long long lint;
typedef vector<lint> vlint;

#define inf 1e+9
#define endl "\\n"
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define rep1(i, n) for (int i = 1; i < (int)(n); i++)
#define rep_rev(i, n) for (int i = n - 1; i >= 0; --i)
#define all(x) (x).begin(), (x).end()

#define pb push_back
#define eb emplace_back
#define Size(c) (int)(c).size()

#define lb(c, x) distance((c).begin(), lower_bound(all(c), (x)))
#define ub(c, x) distance((c).begin(), upper_bound(all(c), (x)))

template <class T> using pq = priority_queue<T>;
template <class T> using pqg = priority_queue<T, vector<T>, greater<T>>;

template <class T, class S> inline bool chmax(T &a, S b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T, class S> inline bool chmin(T &a, S b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> void line(vector<T> &x) {
  for (auto &&xi : x)
    cin >> xi;
}

inline void IN(void) { return; }

template <typename First, typename... Rest>
void IN(First &first, Rest &...rest) {
  cin >> first;
  IN(rest...);
  return;
}

#define INT(...)                                                               \\
  int __VA_ARGS__;                                                             \\
  IN(__VA_ARGS__)
#define LINT(...)                                                              \\
  lint __VA_ARGS__;                                                            \\
  IN(__VA_ARGS__)
#define STR(...)                                                               \\
  string __VA_ARGS__;                                                          \\
  IN(__VA_ARGS__)

template <class T> void UNIQUE(vector<T> &x) {
  sort(all(x));
  x.erase(unique(all(x)), x.end());
}
template <class T> void print_vec(T first, T end) {
  for (auto i = first; i < end - 1; i++)
    cout << *i << " ";
  cout << *(end - 1) << endl;
}

template <class... T> void debug_print(T... args) {
  vector<lint> tmp = initializer_list<lint>{args...};
  print_vec(all(tmp));
}
template <class T> void print(T a) { cout << a << endl; }

vector<string> Yes = {"No", "Yes"};
vector<string> YES = {"NO", "YES"};

int ord(char x) { return (int)(x - 'a'); }
char chr(lint x) { return (char)(x + (lint)('a')); }
lint mod = 1e9 + 7;
// lint mod =

lint sum(vlint a) {
  lint ret = 0;
  for (auto &&v : a)
    ret += v;
  return ret;
}
lint vmini(vlint a, lint &index) {
  lint ret = LLONG_MAX;
  rep(i, Size(a)) {
    if (chmin(ret, a[i]))
      index = i;
  }
  return ret;
}
lint vmaxi(vlint a, lint &index) {
  lint ret = -LLONG_MAX;
  rep(i, Size(a)) {
    if (chmax(ret, a[i]))
      index = i;
  }
  return ret;
}

lint vmin(vlint a) {
  lint ret = LLONG_MAX;
  for (auto &&v : a)
    chmin(ret, v);
  return ret;
}
lint vmax(vlint a) {
  lint ret = -LLONG_MAX;
  for (auto &&v : a)
    chmax(ret, v);
  return ret;
}

int main() {
  INT(n, m);
  vector<vlint> ans;
  vlint check(n + 1, 0);
  check[0] = 1;
  int left = 1, right = n;
  while (Size(ans) < m) {
    if (check[right - left]) {
      left++;
      continue;
    }
    ans.pb({left, right});
    left++;
    right--;
    check[right - left] = 1;
    check[n - right + left] = 1;
  }
  rep(i, m) { print_vec(all(ans[i])); }
}