Language: C++

#include <bits/stdc++.h>
#include <atcoder/all>
using namespace std;
using namespace atcoder;
#define rep2(i, m, n) for (int i = (m); i < (n); ++i)
#define rep(i, n) rep2(i, 0, n)
#define drep2(i, m, n) for (int i = (m)-1; i >= (n); --i)
#define drep(i, n) drep2(i, n, 0)
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
#ifdef LOCAL
void debug_out() { cerr << endl; }
template <typename Head, typename... Tail> void debug_out(Head H, Tail... T) { cerr << ' ' << H; debug_out(T...); }
#define debug(...) cerr << 'L' << __LINE__ << " [" << #__VA_ARGS__ << "]:", debug_out(__VA_ARGS__)
#define dump(x) cerr << 'L' << __LINE__ << " " << #x << " = " << (x) << endl
#else
#define debug(...) (void(0))
#define dump(x) (void(0))
#endif
template<typename T> using V = vector<T>;
template<typename T> using P = pair<T, T>;
using ll = long long;
using ld = long double;
using Vi = V<int>;  using VVi = V<Vi>;
using Vl = V<ll>;   using VVl = V<Vl>;
using Vd = V<ld>;   using VVd = V<Vd>;
using Vb = V<bool>; using VVb = V<Vb>;
using Pi = P<int>;
using Pl = P<ll>;
using Pd = P<ld>;
template<typename T> using priority_queue_rev = priority_queue<T, vector<T>, greater<T>>;
template<typename T> vector<T> make_vec(size_t n, T a) { return vector<T>(n, a); }
template<typename... Ts> auto make_vec(size_t n, Ts... ts) { return vector<decltype(make_vec(ts...))>(n, make_vec(ts...)); }
template<typename T> inline int sz(T &x) { return x.size(); }
template<typename T> inline bool chmin(T &a, T b) { if (a > b) { a = b; return true; } return false; }
template<typename T> inline bool chmax(T &a, T b) { if (a < b) { a = b; return true; } return false; }
template<typename T1, typename T2> istream &operator>>(istream &is, pair<T1, T2> &p) { is >> p.first >> p.second; return is; }
template<typename T1, typename T2> ostream &operator<<(ostream &os, const pair<T1, T2> &p) { os << '(' << p.first << ", " << p.second << ')'; return os; }
template<typename T> istream &operator>>(istream &is, vector<T> &v) { for (auto &e : v) is >> e; return is; }
template<typename T> ostream &operator<<(ostream &os, const vector<T> &v) { for (auto &e : v) os << e << '\n'; return os; }
template<typename T> inline int count_between(vector<T> &a, T l, T r) { return lower_bound(all(a), r) - lower_bound(all(a), l); } // [l, r)
inline ll cDiv(const ll x, const ll y) { return (x+y-1) / y; } // ceil(x/y)
inline int fLog2(const ll x) { assert(x > 0); return 63-__builtin_clzll(x); } // floor(log2(x))
inline int cLog2(const ll x) { assert(x > 0); return (x == 1) ? 0 : 64-__builtin_clzll(x-1); } // ceil(log2(x))
inline int popcount(const ll x) { return __builtin_popcountll(x); }
inline void fail() { cout << -1 << '\n'; exit(0); }
struct fast_ios { fast_ios(){ cin.tie(nullptr); ios::sync_with_stdio(false); cout << fixed << setprecision(20); }; } fast_ios_;
// const int INF  = 1<<30;
// const ll INFll = 1ll<<60;
// const ld EPS   = 1e-10;
// const ld PI    = acos(-1.0);
// const int MOD  = int(1e9)+7;
// const int MOD  = 998244353;

const array<int, 4> dx = {1, 0, -1, 0};
const array<int, 4> dy = {0, 1, 0, -1};

int main() {
  int n, m; cin >> n >> m;
  V<string> S(n); cin >> S;

  auto uid = [&] (int i, int j) { return m*i + j; };

  mf_graph<int> G(n*m+2);
  int s = n*m;
  int t = s + 1;

  rep(i, n) rep(j, m) {
    if (S[i][j] == '#') continue;
    int p = (i+j)&1;
    if (p == 0) {
      rep(d, 4) {
        int i2 = i + dx[d];
        int j2 = j + dy[d];
        if (i2 < 0 || i2 >= n || j2 < 0 || j2 >= m) continue;
        if (S[i2][j2] == '#') continue;
        G.add_edge(uid(i, j), uid(i2, j2), 1);
      }
      G.add_edge(s, uid(i, j), 1);
    }
    else {
      G.add_edge(uid(i, j), t, 1);
    }
  }

  cout << G.flow(s, t) << '\n';

  string rf = "v>^<";

  for (auto [u, v, _, f] : G.edges()) {
    if (u >= s || v >= s) continue;
    if (f != 1) continue;
    int i = u / m;
    int j = u % m;
    int i2 = v / m;
    int j2 = v % m;
    rep(d, 4) {
      if (i2 - i == dx[d] && j2 - j == dy[d]) {
        S[i][j] = rf[d];
        S[i2][j2] = rf[(d+2)%4];
      }
    }
  }
  cout << S;
}

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