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

#include <bits/stdc++.h>
using namespace ::std;
typedef long long lint;
typedef long long ll;
typedef long double ldouble;
typedef vector<lint> vec;
typedef vector<vector<lint>> mat;
typedef vector<vector<vector<lint>>> mat3;
typedef vector<double> dvec;
typedef vector<vector<double>> dmat;
typedef vector<vector<vector<double>>> dmat3;
typedef vector<string> svec;
typedef vector<vector<string>> smat;
typedef vector<vector<vector<string>>> smat3;
#define rep(i, n) for (lint i = 0; i < (lint)(n); i++)
#define irep(i) for (lint i = 0;; i++)
#define irep1(i) for (lint i = 1;; i++)
#define irep2(i) for (lint i = 2;; i++)
#define rrep(i, n) for (lint i = (lint)(n - 1); i > -1; i--)
#define rrepi(i, a, b) for (lint i = (lint)(b - 1); i > a - 1; i--)
#define repi(i, a, b) for (lint i = lint(a); i < lint(b); i++)
#define rep2(i, a, b, c) for (lint i = lint(a); i > lint(b); i += c)
#define all(x) (x).begin(), (x).end()
#define sl(c) (('a') <= (c) && (c) <= ('z'))
#define ll(c) (('A') <= (c) && (c) <= ('Z'))
#define max3(a, b, c) max(a, max(b, c))
#define min3(a, b, c) min(a, min(b, c))
#define PI 3.141592653589793
#define cout (cout << fixed << setprecision(15))
#define makeupper(t) (transform(all(t), t.begin(), ::toupper))
#define makelower(t) (transform(all(t), t.begin(), ::tolower))
#define dist(x1, y1, x2, y2) (pow(pow(x2 - x1, 2) + pow(y2 - y1, 2), 0.5))
#define NEXT_LINE                                                              \\
  string junk;                                                                 \\
  getline(cin, junk);
#define output(v)                                                              \\
  do {                                                                         \\
    rep(i, v.size()) cout << (i ? " " : "") << v[i];                           \\
    cout << "\\n";                                                              \\
  } while (0)
#define output2(v)                                                             \\
  rep(i, v.size()) {                                                           \\
    rep(j, v[i].size()) { cout << (j ? " " : "") << v[i][j]; }                 \\
    cout << "\\n";                                                              \\
  }
#define INF 1LL << 60
#define IINF 1 << 30
#define LINF 9223372036854775807
#define MOD 998244353
vector<lint> dx = {-1, 1, 0, 0, 1, 1, -1, -1};
vector<lint> dy = {0, 0, -1, 1, 1, -1, 1, -1};
vector<lint> operator+(const vector<lint> &v1, const vector<lint> &v2) {
  vector<lint> v(v1.size() + v2.size(), 0);
  rep(i, v1.size()) v[i] = v1[i];
  rep(i, v2.size()) v[i + v1.size()] = v2[i];
  return v;
}
inline lint sum(vector<lint> v) {
  lint sum = 0;
  rep(i, v.size()) sum += v[i];
  return sum;
}
inline string replace(string str, string before, std::string after) {
  string::size_type Pos(str.find(before));
  while (Pos != std::string::npos) {
    str.replace(Pos, before.length(), after);
    Pos = str.find(before, Pos + after.length());
  }
  return str;
}
inline vector<string> split(string s, string delim) {
  vector<string> elems;
  s = replace(s, "#", "HASH");
  s = replace(s, delim, "#");
  stringstream ss(s);
  string item;
  while (getline(ss, item, '#')) {
    elems.push_back(replace(item, "HASH", "#"));
  }
  return elems;
}
inline vector<int> cross(vector<int> a, vector<int> b) {
  return {a[1] * b[2] - a[2] * b[1], a[2] * b[0] - a[0] * b[2],
          a[0] * b[1] - a[1] * b[0]};
}
inline lint GCD(lint a, lint b) { return b ? GCD(b, a % b) : a; }
inline vector<lint> primeList(lint n) {
  vector<bool> p(n + 1);
  vector<lint> list;
  repi(i, 1, n + 1) p[i] = true;
  repi(i, 2, sqrt(n) + 1) {
    if (p[i]) {
      repi(j, 2, n / i + 1) { p[i * j] = false; }
    }
  }
  repi(i, 2, n + 1) if (p[i]) list.push_back(i);
  return list;
}
inline set<lint> primeSet(lint n) {
  vector<bool> p(n + 1);
  set<lint> set;
  repi(i, 1, n + 1) p[i] = true;
  repi(i, 2, sqrt(n) + 1) {
    if (p[i]) {
      repi(j, 2, n / i + 1) { p[i * j] = false; }
    }
  }
  repi(i, 2, n + 1) if (p[i]) set.insert(i);
  return set;
}
inline bool isPrime(int num) {
  if (num < 2)
    return false;
  else if (num == 2)
    return true;
  else if (num % 2 == 0)
    return false;
  double sqrtNum = sqrt(num);
  for (int i = 3; i <= sqrtNum; i += 2) {
    if (num % i == 0)
      return false;
  }
  return true;
}
inline lint max(vector<lint> num) {
  lint M = num[0];
  rep(i, num.size()) M = max(M, num[i]);
  return M;
}
inline lint min(vector<lint> num) {
  lint M = num[0];
  rep(i, num.size()) M = min(M, num[i]);
  return M;
}
inline lint signal(lint val) {
  if (val == 0)
    return 0;
  else
    return val / abs(val);
}
struct IVector2 {
  lint x, y;
};
bool operator<(const IVector2 &s, const IVector2 &t) {
  return s.x == t.x ? s.y < t.y : s.x < t.x;
};
bool operator>(const IVector2 &s, const IVector2 &t) {
  return s.x == t.x ? s.y > t.y : s.x > t.x;
};
struct IVector3 {
  lint x, y, z;
};
struct IVector4 {
  lint x, y, z, w;
};
template <typename... args> struct dfs {
public:
  queue<function<void(void)>> que;
  function<void(args..., dfs<args...> &)> func;
  void run() {
    while (!que.empty()) {
      que.front()();
      que.pop();
    }
  }
  void push(args... arg) { que.push(bind(func, arg..., ref(*this))); }
  dfs(args... init, function<void(args..., dfs<args...> &)> func) : func(func) {
    push(init...);
    run();
  }
};
lint mod_pow(lint x, lint y, lint mod) {
  if (y == 0)
    return 1;
  else if (y == 1)
    return x;
  else if (y % 2 == 1)
    return x * mod_pow(x, y - 1, mod) % mod;
  else
    return mod_pow(x, y / 2, mod) * mod_pow(x, y / 2, mod) % mod;
}
lint binary_search(function<bool(lint)> func) {
  lint left = -1;
  lint right = IINF;
  while (right - left > 1) {
    lint mid = left + (right - left) / 2;
    if (func(mid))
      right = mid;
    else
      left = mid;
  }
  return right;
}

/*std::string RLE(string str)
    {
        string result = "";
        char cmp = str[0];
        lint cnt = 0;
        rep(i, str.length())
        {
            if (cmp == str[i])
            {
                cnt++;
            }
            else
            {
                result += cmp;
                result += cnt;
                cnt = 0;
                cmp = str[i];
            }
        }
    }*/

int main(void) {
  lint n, m;
  cin >> n >> m;
  vector<vec> ans(m, vec(2));
  if (m == 1) {
    cout << "1 2" << endl;
    return 0;
  }
  if (n % 2 == 0) {
    for (lint i = 0; i < m && i + 1 < m; i += 2) {
      ans.at(i) = {i / 2 + 1, n - i / 2};
      ans.at(i + 1) = {n / 2 - i / 2 - 1, n / 2 + 1 + i / 2};
    }
    if (m % 2 != 0) {
      ans.at(m - 1) = {(m - 1) / 2 + 1, n - (m - 1) / 2};
    }
  } else {
    for (lint i = 0; i < m && i + 1 < m; i += 2) {
      ans.at(i) = {i / 2 + 1, n - i / 2};
      ans.at(i + 1) = {n / 2 - i / 2, n / 2 + 2 + i / 2};
    }
    if (m % 2 != 0) {
      ans.at(m - 1) = {(m - 1) / 2 + 1, n - (m - 1) / 2};
    }
  }

  rep(i, m) { cout << ans.at(i).at(0) << ' ' << ans.at(i).at(1) << endl; }
}


Error: WA

Fixed submission:

#include <bits/stdc++.h>
using namespace ::std;
typedef long long lint;
typedef long long ll;
typedef long double ldouble;
typedef vector<lint> vec;
typedef vector<vector<lint>> mat;
typedef vector<vector<vector<lint>>> mat3;
typedef vector<double> dvec;
typedef vector<vector<double>> dmat;
typedef vector<vector<vector<double>>> dmat3;
typedef vector<string> svec;
typedef vector<vector<string>> smat;
typedef vector<vector<vector<string>>> smat3;
#define rep(i, n) for (lint i = 0; i < (lint)(n); i++)
#define irep(i) for (lint i = 0;; i++)
#define irep1(i) for (lint i = 1;; i++)
#define irep2(i) for (lint i = 2;; i++)
#define rrep(i, n) for (lint i = (lint)(n - 1); i > -1; i--)
#define rrepi(i, a, b) for (lint i = (lint)(b - 1); i > a - 1; i--)
#define repi(i, a, b) for (lint i = lint(a); i < lint(b); i++)
#define rep2(i, a, b, c) for (lint i = lint(a); i > lint(b); i += c)
#define all(x) (x).begin(), (x).end()
#define sl(c) (('a') <= (c) && (c) <= ('z'))
#define ll(c) (('A') <= (c) && (c) <= ('Z'))
#define max3(a, b, c) max(a, max(b, c))
#define min3(a, b, c) min(a, min(b, c))
#define PI 3.141592653589793
#define cout (cout << fixed << setprecision(15))
#define makeupper(t) (transform(all(t), t.begin(), ::toupper))
#define makelower(t) (transform(all(t), t.begin(), ::tolower))
#define dist(x1, y1, x2, y2) (pow(pow(x2 - x1, 2) + pow(y2 - y1, 2), 0.5))
#define NEXT_LINE                                                              \\
  string junk;                                                                 \\
  getline(cin, junk);
#define output(v)                                                              \\
  do {                                                                         \\
    rep(i, v.size()) cout << (i ? " " : "") << v[i];                           \\
    cout << "\\n";                                                              \\
  } while (0)
#define output2(v)                                                             \\
  rep(i, v.size()) {                                                           \\
    rep(j, v[i].size()) { cout << (j ? " " : "") << v[i][j]; }                 \\
    cout << "\\n";                                                              \\
  }
#define INF 1LL << 60
#define IINF 1 << 30
#define LINF 9223372036854775807
#define MOD 998244353
vector<lint> dx = {-1, 1, 0, 0, 1, 1, -1, -1};
vector<lint> dy = {0, 0, -1, 1, 1, -1, 1, -1};
vector<lint> operator+(const vector<lint> &v1, const vector<lint> &v2) {
  vector<lint> v(v1.size() + v2.size(), 0);
  rep(i, v1.size()) v[i] = v1[i];
  rep(i, v2.size()) v[i + v1.size()] = v2[i];
  return v;
}
inline lint sum(vector<lint> v) {
  lint sum = 0;
  rep(i, v.size()) sum += v[i];
  return sum;
}
inline string replace(string str, string before, std::string after) {
  string::size_type Pos(str.find(before));
  while (Pos != std::string::npos) {
    str.replace(Pos, before.length(), after);
    Pos = str.find(before, Pos + after.length());
  }
  return str;
}
inline vector<string> split(string s, string delim) {
  vector<string> elems;
  s = replace(s, "#", "HASH");
  s = replace(s, delim, "#");
  stringstream ss(s);
  string item;
  while (getline(ss, item, '#')) {
    elems.push_back(replace(item, "HASH", "#"));
  }
  return elems;
}
inline vector<int> cross(vector<int> a, vector<int> b) {
  return {a[1] * b[2] - a[2] * b[1], a[2] * b[0] - a[0] * b[2],
          a[0] * b[1] - a[1] * b[0]};
}
inline lint GCD(lint a, lint b) { return b ? GCD(b, a % b) : a; }
inline vector<lint> primeList(lint n) {
  vector<bool> p(n + 1);
  vector<lint> list;
  repi(i, 1, n + 1) p[i] = true;
  repi(i, 2, sqrt(n) + 1) {
    if (p[i]) {
      repi(j, 2, n / i + 1) { p[i * j] = false; }
    }
  }
  repi(i, 2, n + 1) if (p[i]) list.push_back(i);
  return list;
}
inline set<lint> primeSet(lint n) {
  vector<bool> p(n + 1);
  set<lint> set;
  repi(i, 1, n + 1) p[i] = true;
  repi(i, 2, sqrt(n) + 1) {
    if (p[i]) {
      repi(j, 2, n / i + 1) { p[i * j] = false; }
    }
  }
  repi(i, 2, n + 1) if (p[i]) set.insert(i);
  return set;
}
inline bool isPrime(int num) {
  if (num < 2)
    return false;
  else if (num == 2)
    return true;
  else if (num % 2 == 0)
    return false;
  double sqrtNum = sqrt(num);
  for (int i = 3; i <= sqrtNum; i += 2) {
    if (num % i == 0)
      return false;
  }
  return true;
}
inline lint max(vector<lint> num) {
  lint M = num[0];
  rep(i, num.size()) M = max(M, num[i]);
  return M;
}
inline lint min(vector<lint> num) {
  lint M = num[0];
  rep(i, num.size()) M = min(M, num[i]);
  return M;
}
inline lint signal(lint val) {
  if (val == 0)
    return 0;
  else
    return val / abs(val);
}
struct IVector2 {
  lint x, y;
};
bool operator<(const IVector2 &s, const IVector2 &t) {
  return s.x == t.x ? s.y < t.y : s.x < t.x;
};
bool operator>(const IVector2 &s, const IVector2 &t) {
  return s.x == t.x ? s.y > t.y : s.x > t.x;
};
struct IVector3 {
  lint x, y, z;
};
struct IVector4 {
  lint x, y, z, w;
};
template <typename... args> struct dfs {
public:
  queue<function<void(void)>> que;
  function<void(args..., dfs<args...> &)> func;
  void run() {
    while (!que.empty()) {
      que.front()();
      que.pop();
    }
  }
  void push(args... arg) { que.push(bind(func, arg..., ref(*this))); }
  dfs(args... init, function<void(args..., dfs<args...> &)> func) : func(func) {
    push(init...);
    run();
  }
};
lint mod_pow(lint x, lint y, lint mod) {
  if (y == 0)
    return 1;
  else if (y == 1)
    return x;
  else if (y % 2 == 1)
    return x * mod_pow(x, y - 1, mod) % mod;
  else
    return mod_pow(x, y / 2, mod) * mod_pow(x, y / 2, mod) % mod;
}
lint binary_search(function<bool(lint)> func) {
  lint left = -1;
  lint right = IINF;
  while (right - left > 1) {
    lint mid = left + (right - left) / 2;
    if (func(mid))
      right = mid;
    else
      left = mid;
  }
  return right;
}

/*std::string RLE(string str)
{
    string result = "";
    char cmp = str[0];
    lint cnt = 0;
    rep(i, str.length())
    {
        if (cmp == str[i])
        {
            cnt++;
        }
        else
        {
            result += cmp;
            result += cnt;
            cnt = 0;
            cmp = str[i];
        }
    }
}*/

int main(void) {
  lint n, m;
  cin >> n >> m;
  vector<vec> ans(m, vec(2));
  if (m == 1) {
    ans.at(0) = {1, 2};
  } else {
    for (lint i = 0; i < m; i = +2) {
      ans.at(i) = {i + 1, n - i};
      ans.at(i + 1) = {n / 2, n / 2 + i};
    }
  }
  rep(i, m) { cout << ans.at(i).at(0) << ' ' << ans.at(i).at(1) << endl; }
}