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

#line 1 "main.cpp"

#line 2 "lib/template.cpp"

#ifndef LOCAL
#pragma GCC diagnostic warning "-w"
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
#pragma GCC target("avx")
#endif
#include <algorithm>
#include <bitset>
#include <cassert>
#include <cmath>
#include <cstring>
#include <functional>
#include <iostream>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <stack>
#include <tuple>
#include <utility>
#include <vector>
using namespace std;
using ll = long long;
using ull = unsigned long long;
using VI = vector<int>;
using VVI = vector<vector<int>>;
using VLL = vector<ll>;
using VVLL = vector<vector<ll>>;
using VB = vector<bool>;
using PII = pair<int, int>;
using PLL = pair<ll, ll>;
template <typename T> using minheap = priority_queue<T, vector<T>, greater<T>>;
constexpr int INF = 1000000007;
constexpr ll INF_LL = 1'000'000'000'000'000'007;
#define EXIT(out)                                                              \\
  do {                                                                         \\
    OUT(out);                                                                  \\
    exit(0);                                                                   \\
  } while (0)
#define all(x) begin(x), end(x)
#define rall(x) rbegin(x), rend(x)
#define newl '\\n'
#define OVERLOAD3(_1, _2, _3, name, ...) name
#define rep(...)                                                               \\
  OVERLOAD3(__VA_ARGS__, REPEAT_FROM_UNTIL, REPEAT_UNTIL, REPEAT)(__VA_ARGS__)
#define REPEAT(times) REPEAT_CNT(_repeat, __COUNTER__, times)
#define REPEAT_CNT(_repeat, cnt, times) REPEAT_CNT_CAT(_repeat, cnt, times)
#define REPEAT_CNT_CAT(_repeat, cnt, times)                                    \\
  REPEAT_FROM_UNTIL(_repeat##cnt, 0, times)
#define REPEAT_UNTIL(name, times) REPEAT_FROM_UNTIL(name, 0, times)
#define REPEAT_FROM_UNTIL(name, from, until)                                   \\
  for (int name = from, name##__until = (until); name < name##__until; name++)
#define repr(...)                                                              \\
  OVERLOAD3(__VA_ARGS__, REPR_FROM_UNTIL, REPR_UNTIL, REPEAT)(__VA_ARGS__)
#define REPR_UNTIL(name, times) REPR_FROM_UNTIL(name, 0, times)
#define REPR_FROM_UNTIL(name, from, until)                                     \\
  for (int name = (until)-1, name##__from = (from); name >= name##__from;      \\
       name--)
#define repi(it, ds) for (auto it = ds.begin(); it != ds.end(); it++)
template <typename T, typename U> bool chmin(T &var, U x) {
  if (var > x) {
    var = x;
    return true;
  } else
    return false;
}
template <typename T, typename U> bool chmax(T &var, U x) {
  if (var < x) {
    var = x;
    return true;
  } else
    return false;
}
template <typename T> int sgn(T val) { return (T(0) < val) - (val < T(0)); }
ll power(ll e, ll t, ll mod = INF_LL) {
  ll res = 1;
  while (t) {
    if (t & 1)
      res = (res * e) % mod;
    t >>= 1;
    e = (e * e) % mod;
  }
  return res;
}
ll choose(ll n, int r) {
  chmin(r, n - r);
  if (r < 0)
    return 0;
  ll res = 1;
  rep(i, r) res *= n - i, res /= i + 1;
  return res;
}
template <typename T> T divceil(T m, T d) {
  assert(m >= 0 and d > 0);
  return (m + d - 1) / d;
}
template <typename T> vector<T> make_v(size_t a, T b) {
  return vector<T>(a, b);
}
template <typename... Ts> auto make_v(size_t a, Ts... ts) {
  return vector<decltype(make_v(ts...))>(a, make_v(ts...));
}
string operator*(const string &s, int times) {
  string res = "";
  rep(times) res += s;
  return res;
}
struct Edge {
  int to;
  ll cost;
  Edge(int _to) : to(_to), cost(1) {}
  Edge(int _to, ll _cost) : to(_to), cost(_cost) {}
  operator int() { return to; }
};
using Graph = vector<vector<Edge>>;
// IO
// formatted version of following is at
// https://shino-sky.github.io/cp-lib/library/formatted_template.cpp.html
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wmisleading-indentation"
class MyScanner {
public:
  int offset = 0;
  char nc() { return getchar(); }
  template <typename T> void input_integer(T &var) {
    var = 0;
    T sign = 1;
    int cc = nc();
    for (; cc < '0' || '9' < cc; cc = nc())
      if (cc == '-')
        sign = -1;
    for (; '0' <= cc && cc <= '9'; cc = nc())
      var = (var << 3) + (var << 1) + cc - '0';
    var = var * sign;
    var += offset;
  }
  int c() {
    char c;
    while (c = nc(), c == ' ' or c == '\\n')
      ;
    return c;
  }
  MyScanner &operator>>(char &var) {
    var = c();
    return *this;
  }
  MyScanner &operator>>(int &var) {
    input_integer<int>(var);
    return *this;
  }
  MyScanner &operator>>(ll &var) {
    input_integer<ll>(var);
    return *this;
  }
  MyScanner &operator>>(string &var) {
    var = "";
    int cc = nc();
    for (; !isgraph(cc); cc = nc())
      ;
    for (; isgraph(cc); cc = nc())
      var.push_back(cc);
    return *this;
  }
  template <size_t N> MyScanner &operator>>(bitset<N> &var) {
    ll v;
    input_integer<ll>(v);
    var = bitset<N>(v);
    return *this;
  }
  template <typename T> operator T() {
    T x;
    *this >> x;
    return x;
  }
  template <typename T> void operator()(T &t) { *this >> t; }
  template <typename T, typename... Ts> void operator()(T &t, Ts &...ts) {
    *this >> t;
    this->operator()(ts...);
  }
  template <typename Iter> void iter(Iter first, Iter last) {
    while (first != last)
      *this >> *first, first++;
  }
  VI vi(int n) {
    VI res(n);
    iter(all(res));
    return res;
  }
  VVI vvi(int n, int m) {
    VVI res(n);
    rep(i, n) res[i] = vi(m);
    return res;
  }
  VLL vll(int n) {
    VLL res(n);
    iter(all(res));
    return res;
  }
  VVLL vvll(int n, int m) {
    VVLL res(n);
    rep(i, n) res[i] = vll(m);
    return res;
  }
  template <typename T> vector<T> v(int n) {
    vector<T> res(n);
    iter(all(res));
    return res;
  }
} IN, IN1{-1};
class MyPrinter {
public:
  int offset = 0;
  template <typename T> void output_integer(T var) {
    var += offset;
    if (var == 0) {
      putchar('0');
      return;
    }
    if (var < 0)
      putchar('-'), var = -var;
    char stack[32];
    int stack_p = 0;
    while (var)
      stack[stack_p++] = '0' + (var % 10), var /= 10;
    while (stack_p)
      putchar(stack[--stack_p]);
  }
  MyPrinter &operator<<(char c) {
    putchar(c);
    return *this;
  }
  MyPrinter &operator<<(double x) {
    printf("%.10f", x);
    return *this;
  }
  template <typename T> MyPrinter &operator<<(T var) {
    output_integer<T>(var);
    return *this;
  }
  MyPrinter &operator<<(char *str_p) {
    while (*str_p)
      putchar(*(str_p++));
    return *this;
  }
  MyPrinter &operator<<(const char *str_p) {
    while (*str_p)
      putchar(*(str_p++));
    return *this;
  }
  MyPrinter &operator<<(const string &str) {
    const char *p = str.c_str();
    const char *l = p + str.size();
    while (p < l)
      putchar(*p++);
    return *this;
  }
  template <typename T> void operator()(T x) { *this << x << newl; }
  template <typename T, typename... Ts> void operator()(T x, Ts... xs) {
    *this << x << " ";
    this->operator()(xs...);
  }
  template <typename Iter> void iter(Iter s, Iter t) {
    if (s == t)
      *this << "\\n";
    else
      for (; s != t; s++)
        *this << *s << " \\n"[next(s, 1) == t];
  }
} OUT, OUT1{1};
template <typename T, typename U>
MyPrinter &operator<<(MyPrinter &out, const pair<T, U> &var) {
  return out << var.first << " " << var.second;
}
template <typename Tuple, size_t I, size_t N, enable_if_t<I == N> * = nullptr>
MyPrinter &tuple_impl(MyPrinter &out, const Tuple &var) {
  return out;
}
template <typename Tuple, size_t I, size_t N, enable_if_t<I != N> * = nullptr>
MyPrinter &tuple_impl(MyPrinter &out, const Tuple &var) {
  out << get<I>(var) << " ";
  return tuple_impl<Tuple, I + 1, N>(out, var);
}
template <typename... Ts>
MyPrinter &operator<<(MyPrinter &out, const tuple<Ts...> &var) {
  return tuple_impl<tuple<Ts...>, 0, sizeof...(Ts)>(out, var);
}
template <typename T, typename U>
MyScanner &operator>>(MyScanner &in, pair<T, U> &var) {
  return in >> var.first >> var.second;
}
template <typename Tuple, size_t I, size_t N, enable_if_t<I == N> * = nullptr>
MyScanner &tuple_impl(MyScanner &in, Tuple &var) {
  return in;
}
template <typename Tuple, size_t I, size_t N, enable_if_t<I != N> * = nullptr>
MyScanner &tuple_impl(MyScanner &in, Tuple &var) {
  in >> get<I>(var);
  return tuple_impl<Tuple, I + 1, N>(in, var);
}
template <typename... Ts>
MyScanner &operator>>(MyScanner &in, tuple<Ts...> &var) {
  return tuple_impl<tuple<Ts...>, 0, sizeof...(Ts)>(in, var);
}
class DebugPrint {
public:
  template <typename T> DebugPrint &operator<<(const T &v) {
#ifdef LOCAL
    cerr << v;
#endif
    return *this;
  }
} debugos;
template <typename T>
DebugPrint &operator<<(DebugPrint &os, const vector<T> &vec) {
  os << "{";
  for (int i = 0; i < vec.size(); i++)
    os << vec[i] << (i + 1 == vec.size() ? "" : ", ");
  os << "}";
  return os;
}
template <typename T, typename U>
DebugPrint &operator<<(DebugPrint &os, map<T, U> &map_var) {
  os << "{";
  repi(itr, map_var) {
    os << *itr;
    itr++;
    if (itr != map_var.end())
      os << ", ";
    itr--;
  }
  os << "}";
  return os;
}
template <typename T> DebugPrint &operator<<(DebugPrint &os, set<T> &set_var) {
  os << "{";
  repi(itr, set_var) {
    os << *itr;
    itr++;
    if (itr != set_var.end())
      os << ", ";
    itr--;
  }
  os << "}";
  return os;
}
template <typename T, typename U>
DebugPrint &operator<<(DebugPrint &os, const pair<T, U> &p) {
  os << "(" << p.first << ", " << p.second << ")";
  return os;
}
void dump_func() { debugos << newl; }
template <class Head, class... Tail>
void dump_func(Head &&head, Tail &&...tail) {
  debugos << head;
  if (sizeof...(Tail) > 0) {
    debugos << ", ";
  }
  dump_func(std::move(tail)...);
}
#define dump(...)                                                              \\
  debugos << "  " << string(#__VA_ARGS__) << ": "                              \\
          << "[" << to_string(__LINE__) << ":" << __FUNCTION__ << "]" << newl  \\
          << "    ",                                                           \\
      dump_func(__VA_ARGS__)
#pragma GCC diagnostic pop

#line 3 "main.cpp"

int main() {
  int n = IN, m = IN;
  if (n % 2 == 1) {
    rep(i, 1, m + 1) OUT(i, n + 1 - i);
  } else {
    VI res(m);
    VB done(n);
    res[0] = n;
    int prev = n;
    done[0] = done[1] = done[n / 2] = true;
    rep(i, 1, m) {
      int now = prev - 1;
      int d = abs(i + 1 - now);
      while (assert(min(d, n - d) != 0), done[min(d, n - d)])
        now--, d = abs(i + 1 - now);
      res[i] = now;
      assert(now > i + 1);
      done[min(d, n - d)] = true;
      prev = now;
    }
    rep(i, m) OUT(i + 1, res[i]);
  }
}


Error: WA

Fixed submission:

#line 1 "main.cpp"

#line 2 "lib/template.cpp"

#ifndef LOCAL
#pragma GCC diagnostic warning "-w"
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
#pragma GCC target("avx")
#endif
#include <algorithm>
#include <bitset>
#include <cassert>
#include <cmath>
#include <cstring>
#include <functional>
#include <iostream>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <stack>
#include <tuple>
#include <utility>
#include <vector>
using namespace std;
using ll = long long;
using ull = unsigned long long;
using VI = vector<int>;
using VVI = vector<vector<int>>;
using VLL = vector<ll>;
using VVLL = vector<vector<ll>>;
using VB = vector<bool>;
using PII = pair<int, int>;
using PLL = pair<ll, ll>;
template <typename T> using minheap = priority_queue<T, vector<T>, greater<T>>;
constexpr int INF = 1000000007;
constexpr ll INF_LL = 1'000'000'000'000'000'007;
#define EXIT(out)                                                              \\
  do {                                                                         \\
    OUT(out);                                                                  \\
    exit(0);                                                                   \\
  } while (0)
#define all(x) begin(x), end(x)
#define rall(x) rbegin(x), rend(x)
#define newl '\\n'
#define OVERLOAD3(_1, _2, _3, name, ...) name
#define rep(...)                                                               \\
  OVERLOAD3(__VA_ARGS__, REPEAT_FROM_UNTIL, REPEAT_UNTIL, REPEAT)(__VA_ARGS__)
#define REPEAT(times) REPEAT_CNT(_repeat, __COUNTER__, times)
#define REPEAT_CNT(_repeat, cnt, times) REPEAT_CNT_CAT(_repeat, cnt, times)
#define REPEAT_CNT_CAT(_repeat, cnt, times)                                    \\
  REPEAT_FROM_UNTIL(_repeat##cnt, 0, times)
#define REPEAT_UNTIL(name, times) REPEAT_FROM_UNTIL(name, 0, times)
#define REPEAT_FROM_UNTIL(name, from, until)                                   \\
  for (int name = from, name##__until = (until); name < name##__until; name++)
#define repr(...)                                                              \\
  OVERLOAD3(__VA_ARGS__, REPR_FROM_UNTIL, REPR_UNTIL, REPEAT)(__VA_ARGS__)
#define REPR_UNTIL(name, times) REPR_FROM_UNTIL(name, 0, times)
#define REPR_FROM_UNTIL(name, from, until)                                     \\
  for (int name = (until)-1, name##__from = (from); name >= name##__from;      \\
       name--)
#define repi(it, ds) for (auto it = ds.begin(); it != ds.end(); it++)
template <typename T, typename U> bool chmin(T &var, U x) {
  if (var > x) {
    var = x;
    return true;
  } else
    return false;
}
template <typename T, typename U> bool chmax(T &var, U x) {
  if (var < x) {
    var = x;
    return true;
  } else
    return false;
}
template <typename T> int sgn(T val) { return (T(0) < val) - (val < T(0)); }
ll power(ll e, ll t, ll mod = INF_LL) {
  ll res = 1;
  while (t) {
    if (t & 1)
      res = (res * e) % mod;
    t >>= 1;
    e = (e * e) % mod;
  }
  return res;
}
ll choose(ll n, int r) {
  chmin(r, n - r);
  if (r < 0)
    return 0;
  ll res = 1;
  rep(i, r) res *= n - i, res /= i + 1;
  return res;
}
template <typename T> T divceil(T m, T d) {
  assert(m >= 0 and d > 0);
  return (m + d - 1) / d;
}
template <typename T> vector<T> make_v(size_t a, T b) {
  return vector<T>(a, b);
}
template <typename... Ts> auto make_v(size_t a, Ts... ts) {
  return vector<decltype(make_v(ts...))>(a, make_v(ts...));
}
string operator*(const string &s, int times) {
  string res = "";
  rep(times) res += s;
  return res;
}
struct Edge {
  int to;
  ll cost;
  Edge(int _to) : to(_to), cost(1) {}
  Edge(int _to, ll _cost) : to(_to), cost(_cost) {}
  operator int() { return to; }
};
using Graph = vector<vector<Edge>>;
// IO
// formatted version of following is at
// https://shino-sky.github.io/cp-lib/library/formatted_template.cpp.html
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wmisleading-indentation"
class MyScanner {
public:
  int offset = 0;
  char nc() { return getchar(); }
  template <typename T> void input_integer(T &var) {
    var = 0;
    T sign = 1;
    int cc = nc();
    for (; cc < '0' || '9' < cc; cc = nc())
      if (cc == '-')
        sign = -1;
    for (; '0' <= cc && cc <= '9'; cc = nc())
      var = (var << 3) + (var << 1) + cc - '0';
    var = var * sign;
    var += offset;
  }
  int c() {
    char c;
    while (c = nc(), c == ' ' or c == '\\n')
      ;
    return c;
  }
  MyScanner &operator>>(char &var) {
    var = c();
    return *this;
  }
  MyScanner &operator>>(int &var) {
    input_integer<int>(var);
    return *this;
  }
  MyScanner &operator>>(ll &var) {
    input_integer<ll>(var);
    return *this;
  }
  MyScanner &operator>>(string &var) {
    var = "";
    int cc = nc();
    for (; !isgraph(cc); cc = nc())
      ;
    for (; isgraph(cc); cc = nc())
      var.push_back(cc);
    return *this;
  }
  template <size_t N> MyScanner &operator>>(bitset<N> &var) {
    ll v;
    input_integer<ll>(v);
    var = bitset<N>(v);
    return *this;
  }
  template <typename T> operator T() {
    T x;
    *this >> x;
    return x;
  }
  template <typename T> void operator()(T &t) { *this >> t; }
  template <typename T, typename... Ts> void operator()(T &t, Ts &...ts) {
    *this >> t;
    this->operator()(ts...);
  }
  template <typename Iter> void iter(Iter first, Iter last) {
    while (first != last)
      *this >> *first, first++;
  }
  VI vi(int n) {
    VI res(n);
    iter(all(res));
    return res;
  }
  VVI vvi(int n, int m) {
    VVI res(n);
    rep(i, n) res[i] = vi(m);
    return res;
  }
  VLL vll(int n) {
    VLL res(n);
    iter(all(res));
    return res;
  }
  VVLL vvll(int n, int m) {
    VVLL res(n);
    rep(i, n) res[i] = vll(m);
    return res;
  }
  template <typename T> vector<T> v(int n) {
    vector<T> res(n);
    iter(all(res));
    return res;
  }
} IN, IN1{-1};
class MyPrinter {
public:
  int offset = 0;
  template <typename T> void output_integer(T var) {
    var += offset;
    if (var == 0) {
      putchar('0');
      return;
    }
    if (var < 0)
      putchar('-'), var = -var;
    char stack[32];
    int stack_p = 0;
    while (var)
      stack[stack_p++] = '0' + (var % 10), var /= 10;
    while (stack_p)
      putchar(stack[--stack_p]);
  }
  MyPrinter &operator<<(char c) {
    putchar(c);
    return *this;
  }
  MyPrinter &operator<<(double x) {
    printf("%.10f", x);
    return *this;
  }
  template <typename T> MyPrinter &operator<<(T var) {
    output_integer<T>(var);
    return *this;
  }
  MyPrinter &operator<<(char *str_p) {
    while (*str_p)
      putchar(*(str_p++));
    return *this;
  }
  MyPrinter &operator<<(const char *str_p) {
    while (*str_p)
      putchar(*(str_p++));
    return *this;
  }
  MyPrinter &operator<<(const string &str) {
    const char *p = str.c_str();
    const char *l = p + str.size();
    while (p < l)
      putchar(*p++);
    return *this;
  }
  template <typename T> void operator()(T x) { *this << x << newl; }
  template <typename T, typename... Ts> void operator()(T x, Ts... xs) {
    *this << x << " ";
    this->operator()(xs...);
  }
  template <typename Iter> void iter(Iter s, Iter t) {
    if (s == t)
      *this << "\\n";
    else
      for (; s != t; s++)
        *this << *s << " \\n"[next(s, 1) == t];
  }
} OUT, OUT1{1};
template <typename T, typename U>
MyPrinter &operator<<(MyPrinter &out, const pair<T, U> &var) {
  return out << var.first << " " << var.second;
}
template <typename Tuple, size_t I, size_t N, enable_if_t<I == N> * = nullptr>
MyPrinter &tuple_impl(MyPrinter &out, const Tuple &var) {
  return out;
}
template <typename Tuple, size_t I, size_t N, enable_if_t<I != N> * = nullptr>
MyPrinter &tuple_impl(MyPrinter &out, const Tuple &var) {
  out << get<I>(var) << " ";
  return tuple_impl<Tuple, I + 1, N>(out, var);
}
template <typename... Ts>
MyPrinter &operator<<(MyPrinter &out, const tuple<Ts...> &var) {
  return tuple_impl<tuple<Ts...>, 0, sizeof...(Ts)>(out, var);
}
template <typename T, typename U>
MyScanner &operator>>(MyScanner &in, pair<T, U> &var) {
  return in >> var.first >> var.second;
}
template <typename Tuple, size_t I, size_t N, enable_if_t<I == N> * = nullptr>
MyScanner &tuple_impl(MyScanner &in, Tuple &var) {
  return in;
}
template <typename Tuple, size_t I, size_t N, enable_if_t<I != N> * = nullptr>
MyScanner &tuple_impl(MyScanner &in, Tuple &var) {
  in >> get<I>(var);
  return tuple_impl<Tuple, I + 1, N>(in, var);
}
template <typename... Ts>
MyScanner &operator>>(MyScanner &in, tuple<Ts...> &var) {
  return tuple_impl<tuple<Ts...>, 0, sizeof...(Ts)>(in, var);
}
class DebugPrint {
public:
  template <typename T> DebugPrint &operator<<(const T &v) {
#ifdef LOCAL
    cerr << v;
#endif
    return *this;
  }
} debugos;
template <typename T>
DebugPrint &operator<<(DebugPrint &os, const vector<T> &vec) {
  os << "{";
  for (int i = 0; i < vec.size(); i++)
    os << vec[i] << (i + 1 == vec.size() ? "" : ", ");
  os << "}";
  return os;
}
template <typename T, typename U>
DebugPrint &operator<<(DebugPrint &os, map<T, U> &map_var) {
  os << "{";
  repi(itr, map_var) {
    os << *itr;
    itr++;
    if (itr != map_var.end())
      os << ", ";
    itr--;
  }
  os << "}";
  return os;
}
template <typename T> DebugPrint &operator<<(DebugPrint &os, set<T> &set_var) {
  os << "{";
  repi(itr, set_var) {
    os << *itr;
    itr++;
    if (itr != set_var.end())
      os << ", ";
    itr--;
  }
  os << "}";
  return os;
}
template <typename T, typename U>
DebugPrint &operator<<(DebugPrint &os, const pair<T, U> &p) {
  os << "(" << p.first << ", " << p.second << ")";
  return os;
}
void dump_func() { debugos << newl; }
template <class Head, class... Tail>
void dump_func(Head &&head, Tail &&...tail) {
  debugos << head;
  if (sizeof...(Tail) > 0) {
    debugos << ", ";
  }
  dump_func(std::move(tail)...);
}
#define dump(...)                                                              \\
  debugos << "  " << string(#__VA_ARGS__) << ": "                              \\
          << "[" << to_string(__LINE__) << ":" << __FUNCTION__ << "]" << newl  \\
          << "    ",                                                           \\
      dump_func(__VA_ARGS__)
#pragma GCC diagnostic pop

#line 3 "main.cpp"

int main() {
  int n = IN, m = IN;
  if (n % 2 == 1) {
    rep(i, 1, m + 1) OUT(i, n + 1 - i);
    assert(false);
  } else {
    VI res(m);
    VB done(n);
    res[0] = n;
    int prev = n;
    done[1] = true;
    rep(i, 1, m) {
      int now = prev - 1;
      int d = abs(i + 1 - now);
      while (assert(min(d, n - d) != 0), done[min(d, n - d)])
        now--, d = abs(i + 1 - now);
      res[i] = now;
      assert(now > i + 1);
      done[min(d, n - d)] = true;
      prev = now;
    }
    rep(i, m) OUT(i + 1, res[i]);
  }
}