Language: C++

#include <bits/stdc++.h>
#include <atcoder/all>
using namespace std;
using namespace atcoder;

//////////////////////////////   Begin Macros

#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
#define in(x, a, b) (a <= x and x < b)
#define rep(i, N) for (int i = 0; i < (int)(N); i++)
#define reprev(i, N) for (int i = (int)(N)-1; i >= 0; i--)
#define rep1(i, N) for (int i = 1; i <= (int)(N); i++)
#define rep1rev(i, N) for (int i = (int)(N); i >= 0; i--)
#define forbe(i, b, e) for (int i = (b); i < (e); i++)
#define forberev(i, b, e) for (int i = (e)-1; i >= (b); i--)
#define forfl(i, f, l) for (int i = (f); i <= (l); i++)
#define forflrev(i, f, l) for (int i = (l); i >= (f); i--)
using ll = long long;
using pii = pair<int, int>;
using pil = pair<int, ll>;
using pli = pair<ll, int>;
using pll = pair<ll, ll>;

template <typename T>
bool chmax(T &m, const T q)
{
  if (m < q)
  {
    m = q;
    return true;
  }
  else
    return false;
}
template <typename T>
bool chmin(T &m, const T q)
{
  if (m > q)
  {
    m = q;
    return true;
  }
  else
    return false;
}
template <typename T1, typename T2>
pair<T1, T2> operator+(const pair<T1, T2> &l, const pair<T1, T2> &r) { return {l.first + r.first, l.second + r.second}; }
template <typename T1, typename T2>
pair<T1, T2> operator-(const pair<T1, T2> &l, const pair<T1, T2> &r) { return {l.first - r.first, l.second - r.second}; }
template <typename T>
pair<T, T> operator*(const pair<T, T> &l, const T &r) { return {l.first * r, l.second * r}; }
template <typename T>
pair<T, T> operator/(const pair<T, T> &l, const T &r) { return {l.first / r, l.second / r}; }
template <typename T>
istream &operator>>(istream &is, vector<T> &vec)
{
  for (auto &v : vec)
    is >> v;
  return is;
}
template <typename T>
ostream &operator<<(ostream &os, const vector<T> &vec)
{
  os << "[";
  for (auto v : vec)
    os << v << ",";
  os << "]";
  return os;
}
template <typename T>
ostream &operator<<(ostream &os, const deque<T> &vec)
{
  os << "deq[";
  for (auto v : vec)
    os << v << ",";
  os << "]";
  return os;
}
template <typename T>
ostream &operator<<(ostream &os, const set<T> &vec)
{
  os << "{";
  for (auto v : vec)
    os << v << ",";
  os << "}";
  return os;
}
template <typename T>
ostream &operator<<(ostream &os, const unordered_set<T> &vec)
{
  os << "{";
  for (auto v : vec)
    os << v << ",";
  os << "}";
  return os;
}
template <typename T>
ostream &operator<<(ostream &os, const multiset<T> &vec)
{
  os << "{";
  for (auto v : vec)
    os << v << ",";
  os << "}";
  return os;
}
template <typename T>
ostream &operator<<(ostream &os, const unordered_multiset<T> &vec)
{
  os << "{";
  for (auto v : vec)
    os << v << ",";
  os << "}";
  return os;
}
template <typename T1, typename T2>
istream &operator>>(istream &is, pair<T1, T2> &pa)
{
  is >> pa.first >> pa.second;
  return is;
}
template <typename T1, typename T2>
ostream &operator<<(ostream &os, const pair<T1, T2> &pa)
{
  os << "(" << pa.first << "," << pa.second << ")";
  return os;
}
template <typename... Ts>
istream &operator>>(istream &is, tuple<Ts...> &theTuple)
{
  apply([&is](Ts &... tupleArgs) { ((is >> tupleArgs), ...); }, theTuple);
  return is;
}
template <typename... Ts>
ostream &operator<<(ostream &os, const tuple<Ts...> &theTuple)
{
  apply([&os](const Ts &... tupleArgs) {
    os << '(';
    size_t n(0);
    ((os << tupleArgs << (++n < sizeof...(Ts) ? "," : "")), ...);
    os << ')';
  },
        theTuple);
  return os;
}
template <typename TK, typename TV>
ostream &operator<<(ostream &os, const map<TK, TV> &mp)
{
  os << "{";
  for (auto v : mp)
    os << v.first << "=>" << v.second << ",";
  os << "}";
  return os;
}
template <typename TK, typename TV>
ostream &operator<<(ostream &os, const unordered_map<TK, TV> &mp)
{
  os << "{";
  for (auto v : mp)
    os << v.first << "=>" << v.second << ",";
  os << "}";
  return os;
}

template <typename T>
void reset(vector<T> &v, const T reset_to)
{
  for (auto &x : v)
    x = reset_to;
}
inline int popcount(const unsigned int x) { return __builtin_popcount(x); }
#define dbg(x) cerr << #x << " = " << (x) << " (L" << __LINE__ << ") " << __FILE__ << endl;

ll nC2(ll n)
{
  return n * (n - 1) / 2;
}

const int intinf = numeric_limits<int>::max();
const ll llinf = numeric_limits<ll>::max();
const pii udlr[4] = {{1, 0}, {0, 1}, {-1, 0}, {0, -1}};

//////////////////////////////   End Macros

void solve()
{
  int n, m;
  cin >> n >> m;
  vector<string> grid(n);
  cin >> grid;

  mf_graph<int> g(n * m + 2);
  int s = n * m, t = n * m + 1;
  rep(i, n) rep(j, m)
  {
    if (grid[i][j] == '#')
      continue;

    int v0 = i * m + j;
    if ((i + j) % 2 == 0)
    {
      g.add_edge(s, v0, 1);
      rep(k, 4)
      {
        auto [di, dj] = udlr[k];
        int ni = i + di, nj = j + dj;
        if (in(ni, 0, n) and in(nj, 0, m) and grid[ni][nj] == '.')
        {
          int v1 = ni * m + nj;
          g.add_edge(v0, v1, 1);
        }
      }
    }
    else
    {
      g.add_edge(v0, t, 1);
    }
  }

  int ans = g.flow(s, t);
  for (auto edge : g.edges())
  {
    if (edge.from == s or edge.to == t or edge.flow == 0)
      continue;
    int i0 = edge.from / m;
    int j0 = edge.from % m;
    int i1 = edge.to / m;
    int j1 = edge.to % m;
    int k;
    rep(k_, 4)
    {
      auto [di, dj] = udlr[k_];
      if (i0 + di == i1 and j0 + dj == j1)
      {
        k = k_;
        break;
      }
    }
    switch (k)
    {
    case 0:
      grid[i0][j0] = 'v';
      grid[i1][j1] = '^';
      break;
    case 1:
      grid[i0][j0] = '>';
      grid[i1][j1] = '<';
      break;
    case 2:
      grid[i0][j0] = '^';
      grid[i1][j1] = 'v';
      break;
    case 3:
      grid[i0][j0] = '<';
      grid[i1][j1] = '>';
      break;
    default:
      break;
    }
  }
  rep(i, n)
  {
    cout << grid[i] << endl;
  }
}

int main()
{
  cerr << "start" << endl;
  // srand(time(0));
  cout << fixed << setprecision(15);

  solve();
  return 0;
}


Compiler output:
a.cc:2:10: fatal error: atcoder/all: No such file or directory
    2 | #include <atcoder/all>
      |          ^~~~~~~~~~~~~
compilation terminated.