Problem p03055 (C++)

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

namespace n91 {

using i8 = std::int_fast8_t;
using i32 = std::int_fast32_t;
using i64 = std::int_fast64_t;
using u8 = std::uint_fast8_t;
using u32 = std::uint_fast32_t;
using u64 = std::uint_fast64_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 <algorithm>
#include <cassert>
#include <iostream>
#include <queue>
#include <string>
#include <utility>

namespace n91 {
bool solve() {
  usize n;
  std::cin >> n;
  auto g = md_vec(n, 0, static_cast<usize>(0));
  for (const auto i : rep(1, n)) {
    usize a, b;
    std::cin >> a >> b;
    --a;
    --b;
    g[a].emplace_back(b);
    g[b].emplace_back(a);
  }
  std::vector<usize> dist(n, 0);
  const auto dfs = [&](const auto &dfs, const usize v, const usize p) -> void {
    for (const auto e : g[v]) {
      if (e != p) {
        dist[e] = dist[v] + 1;
        dfs(dfs, e, v);
      }
    }
  };
  dfs(dfs, 0, n);
  const auto itr = std::max_element(dist.begin(), dist.end());
  *itr = 0;
  dfs(dfs, itr - dist.begin(), n);
  const usize diam = *std::max_element(dist.cbegin(), dist.cend());
  if (diam % 3 == 2) {
    assert(false);
  }
  return diam % 3 == 2;
}

void main_() { std::cout << (solve() ? "First" : "Second") << 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_fast8_t;
using i32 = std::int_fast32_t;
using i64 = std::int_fast64_t;
using u8 = std::uint_fast8_t;
using u32 = std::uint_fast32_t;
using u64 = std::uint_fast64_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 <algorithm>
#include <cassert>
#include <iostream>
#include <queue>
#include <string>
#include <utility>

namespace n91 {
bool solve() {
  usize n;
  std::cin >> n;
  auto g = md_vec(n, 0, static_cast<usize>(0));
  for (const auto i : rep(1, n)) {
    usize a, b;
    std::cin >> a >> b;
    --a;
    --b;
    g[a].emplace_back(b);
    g[b].emplace_back(a);
  }
  std::vector<usize> dist(n, 0);
  const auto dfs = [&](const auto &dfs, const usize v, const usize p) -> void {
    for (const auto e : g[v]) {
      if (e != p) {
        dist[e] = dist[v] + 1;
        dfs(dfs, e, v);
      }
    }
  };
  dfs(dfs, 0, n);
  const auto itr = std::max_element(dist.begin(), dist.end());
  *itr = 0;
  dfs(dfs, itr - dist.begin(), n);
  const usize diam = *std::max_element(dist.cbegin(), dist.cend());
  return diam % 3 == 2 || diam % 3 == 0;
}

void main_() { std::cout << (solve() ? "First" : "Second") << std::endl; }

} // namespace n91

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