Language: C++

#include <bits/stdc++.h>

using namespace std;

int n, m, mx;
vector<vector<char>> grid(1 + n + 1, vector<char>(1 + m + 1, '#'));

struct point {
	int i, j;
  	point() {}
  	point(int _i, int _j)
      :i(_i)
      ,j(_j)
    {}
};

void BFS(point s) {
	vector<int> dist(1 + n + 1, vector<int>(1 + m + 1, -1));
  	queue<int> q;
    q.push(s);
    dist[s.i][s.j] = 0;
  	while (!q.empty()) {
    	int i = q.front().i;
      	int j = q.front().j;
      	q.pop();
        for (int di = -1; di <= 1; ++di)
          	for (int dj = -1; dj <= 1; ++dj) 
            	if (di * di + dj * dj == 1) {
            		int ni = i + di;
                	int nj = j + dj;
                	if (grid[ni][nj] == '#') continue;
              		if (dist[ni][nj] != -1) continue;
              		dist[ni][nj] = dist[i][j] + 1;
                	q.push(point(ni, nj));
            	}
    }
  	for (int i = 1; i <= n; ++i)
      	for (int j = 1; j <= m; ++j)
        	mx = max(mx, dist[i][j]);
  	return;
}

int main() {
  cin >> n >> m;
  
  for (int i = 1; i <= n; ++i)
    	for (int j = 1; j <= m; ++j) cin >> grid[i][j];
  
  for (int i = 1; i <= n; ++i)
    for (int j = 1; j <= m; ++j) 
    	if (grid[i][j] != '#') BFS(point(i, j)));
  
  cout << mx << '\n';
  return 0;
}

Compiler output:
a.cc: In function 'void BFS(point)':
a.cc:18:63: error: no matching function for call to 'std::vector<int>::vector(int, std::vector<int>)'
   18 |         vector<int> dist(1 + n + 1, vector<int>(1 + m + 1, -1));
      |                                                               ^
In file included from /usr/include/c++/14/vector:66,
                 from /usr/include/c++/14/functional:64,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:53,
                 from a.cc:1:
/usr/include/c++/14/bits/stl_vector.h:707:9: note: candidate: 'template<class _InputIterator, class> std::vector<_Tp, _Alloc>::vector(_InputIterator, _InputIterator, const allocator_type&) [with <template-parameter-2-2> = _InputIterator; _Tp = int; _Alloc = std::allocator<int>]'
  707 |         vector(_InputIterator __first, _InputIterator __last,
      |         ^~~~~~
/usr/include/c++/14/bits/stl_vector.h:707:9: note:   template argument deduction/substitution failed:
a.cc:18:63: note:   deduced conflicting types for parameter '_InputIterator' ('int' and 'std::vector<int>')
   18 |         vector<int> dist(1 + n + 1, vector<int>(1 + m + 1, -1));
      |                                                               ^
/usr/include/c++/14/bits/stl_vector.h:678:7: note: candidate: 'std::vector<_Tp, _Alloc>::vector(std::initializer_list<_Tp>, const allocator_type&) [with _Tp = int; _Alloc = std::allocator<int>; allocator_type = std::allocator<int>]'
  678 |       vector(initializer_list<value_type> __l,
      |       ^~~~~~
/usr/include/c++/14/bits/stl_vector.h:678:43: note:   no known conversion for argument 1 from 'int' to 'std::initializer_list<int>'
  678 |       vector(initializer_list<value_type> __l,
      |              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~
/usr/include/c++/14/bits/stl_vector.h:659:7: note: candidate: 'std::vector<_Tp, _Alloc>::vector(std::vector<_Tp, _Alloc>&&, std::__type_identity_t<_Alloc>&) [with _Tp = int; _Alloc = std::allocator<int>; std::__type_identity_t<_Alloc> = std::allocator<int>]'
  659 |       vector(vector&& __rv, const __type_identity_t<allocator_type>& __m)
      |       ^~~~~~
/usr/include/c++/14/bits/stl_vector.h:659:23: note:   no known conversion for argument 1 from 'int' to 'std::vector<int>&&'
  659 |       vector(vector&& __rv, const __type_identity_t<allocator_type>& __m)
      |              ~~~~~~~~~^~~~
/usr/include/c++/14/bits/stl_vector.h:640:7: note: candidate: 'std::vector<_Tp, _Alloc>::vector(std::vector<_Tp, _Alloc>&&, const allocator_type&, std::false_type) [with _Tp = int; _Alloc = std::allocator<int>; allocator_type = std::allocator<int>; std::false_type = std::false_type]'
  640 |       vector(vector&& __rv, const allocator_type& __m, false_type)
      |       ^~~~~~
/usr/include/c++/14/bits/stl_vector.h:640:7: note:   candidate expects 3 arguments, 2 provided
/usr/include/c++/14/bits/stl_vector.h:635:7: note: candidate: 'std::vector<_Tp, _Alloc>::vector(std::vector<_Tp, _Alloc>&&, const allocator_type&, std::true_type) [with _Tp = int; _Alloc = std::allocator<int>; allocator_type = std::allocator<int>; std::true_type = std::true_type]'
  635 |       vector(vector&& __rv, const allocator_type& __m, true_type) noexcept
      |       ^~~~~~
/usr/include/c++/14/bits/stl_vector.h:635:7: note:   candidate expects 3 arguments, 2 provided
/usr/include/c++/14/bits/stl_vector.h:624:7: note: candidate: 'std::vector<_Tp, _Alloc>::vector(const std::vector<_Tp, _Alloc>&, std::__type_identity_t<_Alloc>&) [with _Tp = int; _Alloc = std::allocator<int>; std::__type_identity_t<_Alloc> = std::allocator<int>]'
  624 |       vector(const vector& __x, const __type_identity_t<allocator_type>& __a)
      |       ^~~~~~
/usr/include/c++/14/bits/stl_vector.h:624:28: note:   no known conversion for argument 1 from 'int' to 'const std::vector<int>&'
  624 |       vector(const vector& __x, const __type_identity_t<allocator_type>& __a)
      |              ~~~~~~~~~~~~~~^~~
/usr/include/c++/14/bits/stl_vector.h:620:7: note: candidate: 'std::vector<_Tp, _Alloc>::vector(std::vector<_Tp, _Alloc>&&) [with _Tp = int; _Alloc = std::allocator<int>]'
  620 |       vector(vector&&) noexcept = default;
      |       ^~~~~~
/usr/include/c++/14/bits/stl_vector.h:620:7: note:   candidate expects 1 argument, 2 provided
/usr/include/c++/14/bits/stl_vector.h:601:7: note: candidate: 'std::vector<_Tp, _Alloc>::vector(const std::vector<_Tp, _Alloc>&) [with _Tp = int; _Alloc = std::allocator<int>]'
  601 |       vector(const vector& __x)
      |       ^~~~~~
/usr/include/c++/14/bits/stl_vector.h:601:7: note:   candidate expects 1 argument, 2 provided
/usr/include/c++/14/bits/stl_vector.h:569:7: note: candidate: 'std::vector<_Tp, _Alloc>::vector(size_type, const value_type&, const allocator_type&) [with _Tp = int; _Alloc = std::allocator<int>; size_type = long unsigned int; value_type = int; allocator_type = std::allocator<int>]'
  569 |       vector(size_type __n, const value_type& __value,
      |       ^~~~~~
/usr/include/c++/14/bits/stl_vector.h:569:47: note:   no known conversion for argument 2 from 'std::vector<int>' to 'const std::vector<int>::value_type&' {aka 'const int&'}
  569 |       vector(size_type __n, const value_type& __value,
      |                             ~~~~~~~~~~~~~~~~~~^~~~~~~
/usr/include/c++/14/bits/stl_vector.h:556:7: note: candidate: 'std::vector<_Tp, _Alloc>::vector(size_type, const allocator_type&) [with _Tp = int; _Alloc = std::allocator<int>; size_type = long unsigned int; allocator_type = std::allocator<int>]'
  556 |       vector(size_type __n, const allocator_type& __a = allocator_type())
      |       ^~~~~~
/usr/include/c++/14/bits/stl_vector.h:556:51: note:   no known conversion for argument 2 from 'std::vector<int>' to 'const std::vector<int>::allocator_type&' {aka 'const std::allocator<int>&'}
  556 |       vector(size_type __n, const allocator_type& __a = allocator_type())
      |                             ~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/stl_vector.h:542:7: note: candidate: 'std::vector<_Tp, _Alloc>::vector(const allocator_type&) [with _Tp = int; _Alloc = std::allocator<int>; allocator_type = std::allocator<int>]'
  542 |       vector(const allocator_type& __a) _GLIBCXX_NOEXCEPT
      |       ^~~~~~
/usr/include/c++/14/bits/stl_vector.h:542:7: note:   candidate expects 1 argument, 2 provided
/usr/include/c++/14/bits/stl_vector.h:531:7: note: candidate: 'std::vector<_Tp, _Alloc>::vector() [with _Tp = int; _Alloc = std::allocator<int>]'
  531 |       vector() = default;
      |       ^~~~~~
/usr/include/c++/14/bits/stl_vector.h:531:7: note:   candidate expects 0 arguments, 2 provided
a.cc:20:11: error: no matching function for call to 'std::queue<int>::push(point&)'
   20 |     q.push(s);
      |     ~~~~~~^~~
In file included from /usr/include/c++/14/queue:66,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:157:
/usr/include/c++/14/bits/stl_queue.h:283:7: note: candidate: 'void std::queue<_Tp, _Sequence>::push(const value_type&) [with _Tp = int; _Sequence = std::deque<int, std::allocator<int> >; value_type = int]'
  283 |       push(const value_type& __x)
      |       ^~~~
/usr/include/c++/14/bits/stl_queue.h:283:30: note:   no known conversion for argument 1 from 'point' to 'const std::queue<int>::value_type&' {aka 'const int&'}
  283 |       push(const value_type& __x)
      |            ~~~~~~~~~~~~~~~~~~^~~
/usr/include/c++/14/bits/stl_queue.h:288:7: note: candidate: 'void std::queue<_Tp, _Sequence>::push(value_type&&) [with _Tp = int; _Sequence = std::deque<int, std::allocator<int> >; value_type = int]'
  288 |       push(value_type&& __x)
      |       ^~~~
/usr/include/c++/14/bits/stl_queue.h:288:25: note:   no known conversion for argument 1 from 'point' to 'std::queue<int>::value_type&&' {aka 'int&&'}
  288 |       push(value_type&& __x)
      |            ~~~~~~~~~~~~~^~~
a.cc:21:14: error: invalid types '__gnu_cxx::__alloc_traits<std::allocator<int>, int>::value_type {aka int}[int]' for array subscript
   21 |     dist[s.i][s.j] = 0;
      |              ^
a.cc:23:27: error: request for member 'i' in 'q.std::queue<int>::front()', which is of non-class type '__gnu_cxx::__alloc_traits<std::allocator<int>, int>::value_type' {aka 'int'}
   23 |         int i = q.front().i;
      |                           ^
a.cc:24:27: error: request for member 'j' in 'q.std::queue<int>::front()', which is of non-class type '__gnu_cxx::__alloc_traits<std::allocator<int>, int>::value_type' {aka 'int'}
   24 |         int j = q.front().j;
      |                           ^
a.cc:32:37: error: invalid types '__gnu_cxx::__alloc_traits<std::allocator<int>, int>::value_type {aka int}[int]' for array subscript
   32 |                         if (dist[ni][nj] != -1) continue;
      |                                     ^
a.cc:33:33: error: invalid types '__gnu_cxx::__alloc_traits<std::allocator<int>, int>::value_type {aka int}[int]' for array subscript
   33 |                         dist[ni][nj] = dist[i][j] + 1;
      |                                 ^
a.cc:33:47: error: invalid types '__gnu_cxx::__alloc_traits<std::allocator<int>, int>::value_type {aka int}[int]' for array subscript
   33 |                         dist[ni][nj] = dist[i][j] + 1;
      |                                               ^
a.cc:34:31: error: no matching function for call to 'std::queue<int>::push(point)'
   34 |                         q.push(point(ni, nj));
      |                         ~~~~~~^~~~~~~~~~~~~~~
/usr/include/c++/14/bits/stl_queue.h:283:7: note: candidate: 'void std::queue<_Tp, _Sequence>::push(const value_type&) [with _Tp = int; _Sequence = std::deque<int, std::allocator<int> >; value_type = int]'
  283 |       push(const value_type& __x)
      |       ^~~~
/usr/include/c++/14/bits/stl_queue.h:283:30: note:   no known conversion for argument 1 from 'point' to 'const std::queue<int>::value_type&' {aka 'const int&'}
  283 |       push(const value_type& __x)
      |            ~~~~~~~~~~~~~~~~~~^~~
/usr/include/c++/14/bits/stl_queue.h:288:7: note: candidate: 'void std::queue<_Tp, _Sequence>::push(value_type&&) [with _Tp = int; _Sequence = std::deque<int, std::alloc