Problem p03061 (C++)

Buggy submission (status: Runtime Error):
// #define NDEBUG
#include <cstddef>
#include <cstdint>
#include <vector>

namespace n91 {

using i8 = std::int_least8_t;
using i32 = std::int_least32_t;
using i64 = std::int_least64_t;
using u8 = std::uint_least8_t;
using u32 = std::uint_least32_t;
using u64 = std::uint_least64_t;
using isize = std::ptrdiff_t;
using usize = std::size_t;

class rep {
  const usize f, l;

public:
  class itr {
    friend rep;
    usize i;
    constexpr itr(const usize x) noexcept : i(x) {}

  public:
    void operator++() noexcept { ++i; }
    constexpr usize operator*() const noexcept { return i; }
    constexpr bool operator!=(const itr x) const noexcept { return i != x.i; }
  };
  constexpr rep(const usize first, const usize last) noexcept
      : f(first), l(last) {}
  constexpr itr begin() const noexcept { return itr(f); }
  constexpr itr end() const noexcept { return itr(l); }
};
class revrep {
  const usize f, l;

public:
  class itr {
    friend revrep;
    usize i;
    constexpr itr(usize x) noexcept : i(x) {}

  public:
    void operator++() noexcept { --i; }
    constexpr usize operator*() const noexcept { return i; }
    constexpr bool operator!=(const itr x) const noexcept { return i != x.i; }
  };
  constexpr revrep(usize first, usize last) noexcept : f(--first), l(--last) {}
  constexpr itr begin() const noexcept { return itr(l); }
  constexpr itr end() const noexcept { return itr(f); }
};
template <class T> using vec_alias = std::vector<T>;
template <class T> auto md_vec(const usize n, const T &value) {
  return std::vector<T>(n, value);
}
template <class... Args> auto md_vec(const usize n, Args... args) {
  return std::vector<decltype(md_vec(args...))>(n, md_vec(args...));
}
template <class T> constexpr T difference(const T &a, const T &b) {
  return a < b ? b - a : a - b;
}

} // namespace n91

#include <utility>

namespace n91 {
template <class T> T gcd(T m, T n) {
  while (m %= n)
    ::std::swap(m, n);
  return n;
}
} // namespace n91

#include <algorithm>
#include <iostream>
#include <utility>

namespace n91 {

void main_() {
  usize n;
  std::cin >> n;
  u32 ans = 0;
  std::vector<u32> a(n);
  for (auto &e : a) {
    std::cin >> e;
  }
  std::vector<u32> l(n + 1), r(n + 1);
  l[0] = 0;
  r[n] = 0;
  for (const auto i : rep(0, n)) {
    l[i + 1] = gcd(l[i], a[i]);
  }
  for (const auto i : revrep(0, n)) {
    r[i] = gcd(r[i + 1], a[i]);
  }
  for (const auto i : rep(0, n)) {
    ans = std::max(ans, gcd(l[i], r[i + 1]));
  }
  std::cout << ans << std::endl;
}

} // namespace n91

int main() {
  n91::main_();
  return 0;
}


Fixed submission:
// #define NDEBUG
#include <cstddef>
#include <cstdint>
#include <vector>

namespace n91 {

using i8 = std::int_least8_t;
using i32 = std::int_least32_t;
using i64 = std::int_least64_t;
using u8 = std::uint_least8_t;
using u32 = std::uint_least32_t;
using u64 = std::uint_least64_t;
using isize = std::ptrdiff_t;
using usize = std::size_t;

class rep {
  const usize f, l;

public:
  class itr {
    friend rep;
    usize i;
    constexpr itr(const usize x) noexcept : i(x) {}

  public:
    void operator++() noexcept { ++i; }
    constexpr usize operator*() const noexcept { return i; }
    constexpr bool operator!=(const itr x) const noexcept { return i != x.i; }
  };
  constexpr rep(const usize first, const usize last) noexcept
      : f(first), l(last) {}
  constexpr itr begin() const noexcept { return itr(f); }
  constexpr itr end() const noexcept { return itr(l); }
};
class revrep {
  const usize f, l;

public:
  class itr {
    friend revrep;
    usize i;
    constexpr itr(usize x) noexcept : i(x) {}

  public:
    void operator++() noexcept { --i; }
    constexpr usize operator*() const noexcept { return i; }
    constexpr bool operator!=(const itr x) const noexcept { return i != x.i; }
  };
  constexpr revrep(usize first, usize last) noexcept : f(--first), l(--last) {}
  constexpr itr begin() const noexcept { return itr(l); }
  constexpr itr end() const noexcept { return itr(f); }
};
template <class T> using vec_alias = std::vector<T>;
template <class T> auto md_vec(const usize n, const T &value) {
  return std::vector<T>(n, value);
}
template <class... Args> auto md_vec(const usize n, Args... args) {
  return std::vector<decltype(md_vec(args...))>(n, md_vec(args...));
}
template <class T> constexpr T difference(const T &a, const T &b) {
  return a < b ? b - a : a - b;
}

} // namespace n91

#include <utility>

namespace n91 {
template <class T> T gcd(T m, T n) {
  if (n == 0) {
    return m;
  }
  while (m %= n)
    ::std::swap(m, n);
  return n;
}
} // namespace n91

#include <algorithm>
#include <iostream>
#include <utility>

namespace n91 {

void main_() {
  usize n;
  std::cin >> n;
  u32 ans = 0;
  std::vector<u32> a(n);
  for (auto &e : a) {
    std::cin >> e;
  }
  std::vector<u32> l(n + 1), r(n + 1);
  l[0] = 0;
  r[n] = 0;
  for (const auto i : rep(0, n)) {
    l[i + 1] = gcd(l[i], a[i]);
  }
  for (const auto i : revrep(0, n)) {
    r[i] = gcd(r[i + 1], a[i]);
  }
  for (const auto i : rep(0, n)) {
    ans = std::max(ans, gcd(l[i], r[i + 1]));
  }
  std::cout << ans << std::endl;
}

} // namespace n91

int main() {
  n91::main_();
  return 0;
}