Problem p03506 (C++)

Buggy submission (status: Time Limit Exceeded):
#include "bits/stdc++.h"
// Begin Header {{{
#define all(x) (x).begin(), (x).end()
#define rep(i, s, n) for (i64 i = (s), i##_limit = (n); i < i##_limit; ++i)
#define repr(i, s, t) for (i64 i = (s), i##_limit = (t); i >= i##_limit; --i)
#define VAR(type, ...)                                                         \
  type __VA_ARGS__;                                                            \
  read(__VA_ARGS__);
#ifndef DBG
#define dump(...)
#endif
using namespace std;
using i64 = int_fast64_t;
using pii = pair<i64, i64>;
template <class T, class U> inline bool chmax(T &a, const U &b) {
  return b > a && (a = b, true);
}
template <class T, class U> inline bool chmin(T &a, const U &b) {
  return b < a && (a = b, true);
}
constexpr int INF = 0x3f3f3f3f;
constexpr i64 LINF = 0x3f3f3f3f3f3f3f3fLL;

template <class T> inline vector<T> makeV(const T &initValue, size_t sz) {
  return vector<T>(sz, initValue);
}

template <class T, class... Args>
inline auto makeV(const T &initValue, size_t sz, Args... args) {
  return vector<decltype(makeV<T>(initValue, args...))>(
      sz, makeV<T>(initValue, args...));
}

template <class T> inline istream &operator>>(istream &is, vector<T> &vec) {
  for (auto &e : vec)
    is >> e;
  return is;
}

inline void read() {}

template <class Head, class... Tail>
inline void read(Head &head, Tail &...tail) {
  cin >> head;
  read(tail...);
}

inline void print() { cout << "\n"; }

template <class Head, class... Tail>
inline void print(Head &&head, Tail &&...tail) {
  cout << head;
  if (sizeof...(tail))
    cout << ' ';
  print(forward<Tail>(tail)...);
}

template <class T>
inline ostream &operator<<(ostream &os, const vector<T> &vec) {
  static constexpr const char *delim[] = {" ", ""};
  for (const auto &e : vec)
    os << e << delim[&e == &vec.back()];
  return os;
}

template <class Container> struct Rev {
  Container &x_;
  inline Rev(Container &x) : x_(x) {}
  inline auto begin() { return rbegin(x_); }
  inline auto end() { return rend(x_); }
};
// }}} End Header

int N;

inline i64 parent(i64 u) { return (u - 1) / N; }

inline i64 depth(i64 u) {
  i64 d = 0;
  while (u > 0)
    ++d, u = parent(u);
  return d;
}

inline i64 findLCA(i64 a, i64 b) {
  while (a != b) {
    const i64 depA = depth(a);
    const i64 depB = depth(b);
    if (depA > depB) {
      a = parent(a);
    } else if (depA < depB) {
      b = parent(b);
    } else {
      a = parent(a);
      b = parent(b);
    }
  }
  return a;
}

signed main() {
  ios_base::sync_with_stdio(false);
  cin.tie(nullptr);

  int Q;
  read(N, Q);

  while (Q--) {
    VAR(int, a, b);
    --a, --b;
    print(findLCA(a, b) + 1);
  }

  return 0;
}


Fixed submission:
#include "bits/stdc++.h"
// Begin Header {{{
#define all(x) (x).begin(), (x).end()
#define rep(i, s, n) for (i64 i = (s), i##_limit = (n); i < i##_limit; ++i)
#define repr(i, s, t) for (i64 i = (s), i##_limit = (t); i >= i##_limit; --i)
#define VAR(type, ...)                                                         \
  type __VA_ARGS__;                                                            \
  read(__VA_ARGS__);
#ifndef DBG
#define dump(...)
#endif
using namespace std;
using i64 = int_fast64_t;
using pii = pair<i64, i64>;
template <class T, class U> inline bool chmax(T &a, const U &b) {
  return b > a && (a = b, true);
}
template <class T, class U> inline bool chmin(T &a, const U &b) {
  return b < a && (a = b, true);
}
constexpr int INF = 0x3f3f3f3f;
constexpr i64 LINF = 0x3f3f3f3f3f3f3f3fLL;

template <class T> inline vector<T> makeV(const T &initValue, size_t sz) {
  return vector<T>(sz, initValue);
}

template <class T, class... Args>
inline auto makeV(const T &initValue, size_t sz, Args... args) {
  return vector<decltype(makeV<T>(initValue, args...))>(
      sz, makeV<T>(initValue, args...));
}

template <class T> inline istream &operator>>(istream &is, vector<T> &vec) {
  for (auto &e : vec)
    is >> e;
  return is;
}

inline void read() {}

template <class Head, class... Tail>
inline void read(Head &head, Tail &...tail) {
  cin >> head;
  read(tail...);
}

inline void print() { cout << "\n"; }

template <class Head, class... Tail>
inline void print(Head &&head, Tail &&...tail) {
  cout << head;
  if (sizeof...(tail))
    cout << ' ';
  print(forward<Tail>(tail)...);
}

template <class T>
inline ostream &operator<<(ostream &os, const vector<T> &vec) {
  static constexpr const char *delim[] = {" ", ""};
  for (const auto &e : vec)
    os << e << delim[&e == &vec.back()];
  return os;
}

template <class Container> struct Rev {
  Container &x_;
  inline Rev(Container &x) : x_(x) {}
  inline auto begin() { return rbegin(x_); }
  inline auto end() { return rend(x_); }
};
// }}} End Header

int N;

inline i64 parent(i64 u) { return (u - 1) / N; }

inline i64 depth(i64 u) {
  i64 d = 0;
  while (u > 0)
    ++d, u = parent(u);
  return d;
}

inline i64 findLCA(i64 a, i64 b) {
  if (N == 1) {
    return min(a, b);
  }
  while (a != b) {
    const i64 depA = depth(a);
    const i64 depB = depth(b);
    if (depA > depB) {
      a = parent(a);
    } else if (depA < depB) {
      b = parent(b);
    } else {
      a = parent(a);
      b = parent(b);
    }
  }
  return a;
}

signed main() {
  ios_base::sync_with_stdio(false);
  cin.tie(nullptr);

  int Q;
  read(N, Q);

  while (Q--) {
    VAR(int, a, b);
    --a, --b;
    print(findLCA(a, b) + 1);
  }

  return 0;
}