Problem p03429 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
using namespace std;

int n, m, a, b;
vector<vector<char>> ans(1010, vector<char>(1010, '.'));

void answer() {
  if (a + b) {
    cout << "NO" << endl;
  } else {
    cout << "YES" << endl;
    for (int i = 0; i < n; ++i) {
      for (int j = 0; j < m; ++j) {
        cout << ans[i][j];
      }
      cout << endl;
    }
  }
}

bool puta(int y, int x) {
  if (ans[y][x] != '.' || ans[y][x + 1] != '.')
    return false;
  if (!a)
    return false;
  ans[y][x] = '<';
  ans[y][x + 1] = '>';
  a--;
  return true;
}

bool putb(int y, int x) {
  if (ans[y][x] != '.' || ans[y + 1][x] != '.')
    return false;
  if (!b)
    return false;
  ans[y][x] = '^';
  ans[y + 1][x] = 'v';
  b--;
  return true;
}

int main() {
  cin >> n >> m >> a >> b;

  int N = n - (n & 1), M = m - (m & 1);

  if (n * m & 1) {
    for (int j = 0; j < m - 3; j += 2) {
      puta(n - 1, j);
    }
    for (int i = 0; i < n - 3; i += 2) {
      putb(i, m - 1);
    }

    puta(n - 1, m - 2);
    putb(n - 3, m - 1);
    if (a * b & 1) {
      puta(n - 3, m - 3);
      putb(n - 2, m - 3);
    }
  } else {

    if (n & 1) {
      for (int j = 0; a && j < M; j += 2) {
        puta(N, j);
      }
    }
    if (m & 1) {
      for (int i = 0; b && i < N; i += 2) {
        putb(i, M);
      }
    }
  }

  for (int i = 0; i < N; i += 2) {
    for (int j = 0; j < M; j += 2) {
      if (a) {
        puta(i, j);
        if (a)
          puta(i + 1, j);
      } else if (b) {
        putb(i, j);
        if (b)
          putb(i, j + 1);
      }
    }
  }

  answer();
}


Fixed submission:
#include <bits/stdc++.h>
using namespace std;

int n, m, a, b;
vector<vector<char>> ans(1010, vector<char>(1010, '.'));

void answer() {
  if (a + b) {
    cout << "NO" << endl;
  } else {
    cout << "YES" << endl;
    for (int i = 0; i < n; ++i) {
      for (int j = 0; j < m; ++j) {
        cout << ans[i][j];
      }
      cout << endl;
    }
  }
}

bool puta(int y, int x) {
  if (ans[y][x] != '.' || ans[y][x + 1] != '.')
    return false;
  if (!a)
    return false;
  ans[y][x] = '<';
  ans[y][x + 1] = '>';
  a--;
  return true;
}

bool putb(int y, int x) {
  if (ans[y][x] != '.' || ans[y + 1][x] != '.')
    return false;
  if (!b)
    return false;
  ans[y][x] = '^';
  ans[y + 1][x] = 'v';
  b--;
  return true;
}

int main() {
  cin >> n >> m >> a >> b;
  if (n == 1 && m == 1) {
    answer();
    return 0;
  }

  int N = n - (n & 1), M = m - (m & 1);

  if (n * m & 1) {
    for (int j = 0; j < m - 3; j += 2) {
      puta(n - 1, j);
    }
    for (int i = 0; i < n - 3; i += 2) {
      putb(i, m - 1);
    }

    puta(n - 1, m - 2);
    putb(n - 3, m - 1);
    if (a * b & 1) {
      puta(n - 3, m - 3);
      putb(n - 2, m - 3);
    }
  } else {

    if (n & 1) {
      for (int j = 0; a && j < M; j += 2) {
        puta(N, j);
      }
    }
    if (m & 1) {
      for (int i = 0; b && i < N; i += 2) {
        putb(i, M);
      }
    }
  }

  for (int i = 0; i < N; i += 2) {
    for (int j = 0; j < M; j += 2) {
      if (a) {
        puta(i, j);
        if (a)
          puta(i + 1, j);
      } else if (b) {
        putb(i, j);
        if (b)
          putb(i, j + 1);
      }
    }
  }

  answer();
}