Problem p03429 (C++)

Buggy submission (status: Runtime Error):
#include <algorithm>
#include <cmath>
#include <iomanip>
#include <iostream>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <string>
#define _USE_MATH_DEFINES
#include <complex>
#include <functional>
#include <math.h>
using namespace std;

#define rep(i, x) for (ll i = 0; i < x; i++)
#define repn(i, x) for (ll i = 1; i <= x; i++)

typedef long long ll;
const ll INF = 1e17;
const ll MOD = 998244353;
const ll MAX = 4000001;
const long double eps = 1E-14;

ll max(ll a, ll b) {
  if (a > b) {
    return a;
  }
  return b;
}

ll min(ll a, ll b) {
  if (a > b) {
    return b;
  }
  return a;
}

ll gcd(ll a, ll b) {
  if (b == 0) {
    return a;
  }
  if (a < b) {
    return gcd(b, a);
  }
  return gcd(b, a % b);
}

ll lcm(ll a, ll b) { return a / gcd(a, b) * b; }

struct edge {
  ll ind;
  ll fr;
  ll to;
  ll d;
};

class mint {
  long long x;

public:
  mint(long long x = 0) : x((x % MOD + MOD) % MOD) {}
  mint operator-() const { return mint(-x); }
  mint &operator+=(const mint &a) {
    if ((x += a.x) >= MOD)
      x -= MOD;
    return *this;
  }
  mint &operator-=(const mint &a) {
    if ((x += MOD - a.x) >= MOD)
      x -= MOD;
    return *this;
  }
  mint &operator*=(const mint &a) {
    (x *= a.x) %= MOD;
    return *this;
  }
  mint operator+(const mint &a) const {
    mint res(*this);
    return res += a;
  }
  mint operator-(const mint &a) const {
    mint res(*this);
    return res -= a;
  }
  mint operator*(const mint &a) const {
    mint res(*this);
    return res *= a;
  }

  mint pow(ll t) const {
    if (!t)
      return 1;
    mint a = pow(t >> 1);
    a *= a;
    if (t & 1)
      a *= *this;
    return a;
  }

  // for prime MOD
  mint inv() const { return pow(MOD - 2); }
  mint &operator/=(const mint &a) { return (*this) *= a.inv(); }
  mint operator/(const mint &a) const {
    mint res(*this);
    return res /= a;
  }

  friend ostream &operator<<(ostream &os, const mint &m) {
    os << m.x;
    return os;
  }
};

mint pw(mint a, ll b) {
  if (b == 0) {
    return 1;
  }
  mint ret = pw(a, b >> 1);
  ret *= ret;
  if (b & 1) {
    ret *= a;
  }
  return ret;
}

typedef vector<ll> vll;
typedef vector<vector<ll>> vvll;
typedef vector<vector<vector<ll>>> vvvll;

typedef vector<mint> vmint;
typedef vector<vector<mint>> vvmint;
typedef vector<vector<vector<mint>>> vvvmint;

//////////////////////////////////////

pair<bool, vector<vector<char>>> nuri(ll N, ll M, ll A, ll B, bool b) {
  vector<vector<char>> ans(N + 1, vector<char>(M + 1, '.'));

  ll x = A;
  ll y = B;

  if (N % 2 == 1) {
    repn(j, M / 2) {
      if (b && 2 * j + 1 == M) {
        continue;
      }
      if (x >= 1) {
        ans[N][2 * j - 1] = '<';
        ans[N][2 * j] = '>';
        x--;
      }
    }
    // repn(k, N) {
    //	repn(l, M)cout << ans[k][l];
    //	cout << endl;
    // }
    // cout << endl;
  }

  if (M % 2 == 1) {
    repn(i, N / 2) {
      if (b && 2 * i + 1 == N) {
        continue;
      }
      if (y >= 1) {
        ans[2 * i - 1][M] = '^';
        ans[2 * i][M] = 'v';
        y--;
      }
    }
  }

  repn(i, N / 2) repn(j, M / 2) {
    if (b && 2 * i + 1 == N && 2 * j + 1 == M) {
      continue;
    }

    if (x >= y) {
      if (x >= 1) {
        ans[2 * i - 1][2 * j - 1] = '<';
        ans[2 * i - 1][2 * j] = '>';
        x--;
      }
      if (x >= 1) {
        ans[2 * i][2 * j - 1] = '<';
        ans[2 * i][2 * j] = '>';
        x--;
      }
    }

    else {
      if (y >= 1) {
        ans[2 * i - 1][2 * j - 1] = '^';
        ans[2 * i][2 * j - 1] = 'v';
        y--;
      }
      if (y >= 1) {
        ans[2 * i - 1][2 * j] = '^';
        ans[2 * i][2 * j] = 'v';
        y--;
      }
    }

    // repn(k, N) {
    //	repn(l, M)cout << ans[k][l];
    //	cout << endl;
    // }
    // cout << x << " " << y << endl;
    // cout << endl;
  }

  if (!b) {
    if (x > 0 || y > 0) {
      return {0, ans};
    }
    return {1, ans};
  }

  if (x >= 3 || y >= 3) {
    return nuri(N, M, A, B, 0);
  }

  if (N % 2 == 1 && M % 2 == 1) {
    if (y >= 1) {
      ans[N - 2][M - 2] = '^';
      ans[N - 1][M - 2] = 'v';
      y -= 1;
    }
    if (y >= 1) {
      ans[N - 1][M] = '^';
      ans[N][M] = 'v';
      y -= 1;
    }
    if (x >= 1) {
      ans[N - 2][M - 1] = '<';
      ans[N - 2][M] = '>';
      x -= 1;
    }
    if (x >= 1) {
      ans[N][M - 2] = '<';
      ans[N][M - 1] = '>';
      x -= 1;
    }
  }

  if (x > 0 || y > 0) {
    return {0, ans};
  } else {
    return {1, ans};
  }
}

int main() {

  ll N, M, A, B;
  cin >> N >> M >> A >> B;

  auto c = nuri(N, M, A, B, 0);
  if (N > 1 || M > 1) {
    c = nuri(N, M, A, B, 1);
  }

  if (c.first == 0) {
    cout << "NO" << endl;
    return 0;
  }
  cout << "YES" << endl;

  repn(i, N) {
    repn(j, M) { cout << c.second[i][j]; }
    cout << endl;
  }
}

Fixed submission:
#include <algorithm>
#include <cmath>
#include <iomanip>
#include <iostream>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <string>
#define _USE_MATH_DEFINES
#include <complex>
#include <functional>
#include <math.h>
using namespace std;

#define rep(i, x) for (ll i = 0; i < x; i++)
#define repn(i, x) for (ll i = 1; i <= x; i++)

typedef long long ll;
const ll INF = 1e17;
const ll MOD = 998244353;
const ll MAX = 4000001;
const long double eps = 1E-14;

ll max(ll a, ll b) {
  if (a > b) {
    return a;
  }
  return b;
}

ll min(ll a, ll b) {
  if (a > b) {
    return b;
  }
  return a;
}

ll gcd(ll a, ll b) {
  if (b == 0) {
    return a;
  }
  if (a < b) {
    return gcd(b, a);
  }
  return gcd(b, a % b);
}

ll lcm(ll a, ll b) { return a / gcd(a, b) * b; }

struct edge {
  ll ind;
  ll fr;
  ll to;
  ll d;
};

class mint {
  long long x;

public:
  mint(long long x = 0) : x((x % MOD + MOD) % MOD) {}
  mint operator-() const { return mint(-x); }
  mint &operator+=(const mint &a) {
    if ((x += a.x) >= MOD)
      x -= MOD;
    return *this;
  }
  mint &operator-=(const mint &a) {
    if ((x += MOD - a.x) >= MOD)
      x -= MOD;
    return *this;
  }
  mint &operator*=(const mint &a) {
    (x *= a.x) %= MOD;
    return *this;
  }
  mint operator+(const mint &a) const {
    mint res(*this);
    return res += a;
  }
  mint operator-(const mint &a) const {
    mint res(*this);
    return res -= a;
  }
  mint operator*(const mint &a) const {
    mint res(*this);
    return res *= a;
  }

  mint pow(ll t) const {
    if (!t)
      return 1;
    mint a = pow(t >> 1);
    a *= a;
    if (t & 1)
      a *= *this;
    return a;
  }

  // for prime MOD
  mint inv() const { return pow(MOD - 2); }
  mint &operator/=(const mint &a) { return (*this) *= a.inv(); }
  mint operator/(const mint &a) const {
    mint res(*this);
    return res /= a;
  }

  friend ostream &operator<<(ostream &os, const mint &m) {
    os << m.x;
    return os;
  }
};

mint pw(mint a, ll b) {
  if (b == 0) {
    return 1;
  }
  mint ret = pw(a, b >> 1);
  ret *= ret;
  if (b & 1) {
    ret *= a;
  }
  return ret;
}

typedef vector<ll> vll;
typedef vector<vector<ll>> vvll;
typedef vector<vector<vector<ll>>> vvvll;

typedef vector<mint> vmint;
typedef vector<vector<mint>> vvmint;
typedef vector<vector<vector<mint>>> vvvmint;

//////////////////////////////////////

pair<bool, vector<vector<char>>> nuri(ll N, ll M, ll A, ll B, bool b) {
  vector<vector<char>> ans(N + 1, vector<char>(M + 1, '.'));

  ll x = A;
  ll y = B;

  if (N % 2 == 1) {
    repn(j, M / 2) {
      if (b && 2 * j + 1 == M) {
        continue;
      }
      if (x >= 1) {
        ans[N][2 * j - 1] = '<';
        ans[N][2 * j] = '>';
        x--;
      }
    }
    // repn(k, N) {
    //	repn(l, M)cout << ans[k][l];
    //	cout << endl;
    // }
    // cout << endl;
  }

  if (M % 2 == 1) {
    repn(i, N / 2) {
      if (b && 2 * i + 1 == N) {
        continue;
      }
      if (y >= 1) {
        ans[2 * i - 1][M] = '^';
        ans[2 * i][M] = 'v';
        y--;
      }
    }
  }

  repn(i, N / 2) repn(j, M / 2) {
    if (b && 2 * i + 1 == N && 2 * j + 1 == M) {
      continue;
    }

    if (x >= y) {
      if (x >= 1) {
        ans[2 * i - 1][2 * j - 1] = '<';
        ans[2 * i - 1][2 * j] = '>';
        x--;
      }
      if (x >= 1) {
        ans[2 * i][2 * j - 1] = '<';
        ans[2 * i][2 * j] = '>';
        x--;
      }
    }

    else {
      if (y >= 1) {
        ans[2 * i - 1][2 * j - 1] = '^';
        ans[2 * i][2 * j - 1] = 'v';
        y--;
      }
      if (y >= 1) {
        ans[2 * i - 1][2 * j] = '^';
        ans[2 * i][2 * j] = 'v';
        y--;
      }
    }

    // repn(k, N) {
    //	repn(l, M)cout << ans[k][l];
    //	cout << endl;
    // }
    // cout << x << " " << y << endl;
    // cout << endl;
  }

  if (!b) {
    if (x > 0 || y > 0) {
      return {0, ans};
    }
    return {1, ans};
  }

  if (x >= 3 || y >= 3) {
    return nuri(N, M, A, B, 0);
  }

  if (N % 2 == 1 && M % 2 == 1) {
    if (y >= 1) {
      ans[N - 2][M - 2] = '^';
      ans[N - 1][M - 2] = 'v';
      y -= 1;
    }
    if (y >= 1) {
      ans[N - 1][M] = '^';
      ans[N][M] = 'v';
      y -= 1;
    }
    if (x >= 1) {
      ans[N - 2][M - 1] = '<';
      ans[N - 2][M] = '>';
      x -= 1;
    }
    if (x >= 1) {
      ans[N][M - 2] = '<';
      ans[N][M - 1] = '>';
      x -= 1;
    }
  }

  if (x > 0 || y > 0) {
    return {0, ans};
  } else {
    return {1, ans};
  }
}

int main() {

  ll N, M, A, B;
  cin >> N >> M >> A >> B;

  auto c = nuri(N, M, A, B, 0);
  if (N > 1 && M > 1) {
    c = nuri(N, M, A, B, 1);
  }

  if (c.first == 0) {
    cout << "NO" << endl;
    return 0;
  }
  cout << "YES" << endl;

  repn(i, N) {
    repn(j, M) { cout << c.second[i][j]; }
    cout << endl;
  }
}