Language: C++

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

int main() {
	int h,w;
	int dx[4] = {1, -1, 0, 0};
	int dy[4] = {0, 0, 1, -1};
	cin >> h >> w;
	char grid[20][20];
	for(int i=0; i<h; i++){
		for(int j=0; j<w; j++){
			cin >> grid[i][j];
		}
	}
	long long ans = 0;
	for(int i=0; i<h; i++){
		for(int j=0; j<w; j++){

			if(grid[i][j] == '#'){
				continue;
			}else{
				int board[20][20];
				for(int ii=0; ii<h; ii++){
					for(int jj=0; jj<w; jj++){
						board[ii][jj] = -1;
					}
				}

				board[i][j] = 0;
				queue<int> queueX;
				queue<int> queueY;
				queueX.push(j);
				queueY.push(i);

				while(queueX.size() > 0){
					int x = queueX.front();
					int y = queueY.front();
					queueX.pop();
					queueY.pop();
					for(int ii=0; ii<4; ii++){
						int nextX =x + dx[ii];
						int nextY =y + dy[ii];

						if(0<=nextX && nextX<=w && 0<=nextY && nextY<=h && board[nextY][nextX] == -1 && grid[nextY][nextX] == '.'){
							board[nextY][nextX] = board[y][x] + 1;
							queueX.push(nextX);
							queueY.push(nextY);
						}
					}
				}
				for(int ii=0; ii<h; ii++){
					for(int jj=0; jj<w; jj++){
						ans = max(ans, board[ii][jj]);
					}
				}
			}
		}
	}
	cout << ans << endl;
}

Compiler output:
a.cc: In function 'int main()':
a.cc:53:58: error: no matching function for call to 'max(long long int&, int&)'
   53 |                                                 ans = max(ans, board[ii][jj]);
      |                                                       ~~~^~~~~~~~~~~~~~~~~~~~
In file included from /usr/include/c++/14/algorithm:60,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:51,
                 from a.cc:1:
/usr/include/c++/14/bits/stl_algobase.h:257:5: note: candidate: 'template<class _Tp> constexpr const _Tp& std::max(const _Tp&, const _Tp&)'
  257 |     max(const _Tp& __a, const _Tp& __b)
      |     ^~~
/usr/include/c++/14/bits/stl_algobase.h:257:5: note:   template argument deduction/substitution failed:
a.cc:53:58: note:   deduced conflicting types for parameter 'const _Tp' ('long long int' and 'int')
   53 |                                                 ans = max(ans, board[ii][jj]);
      |                                                       ~~~^~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/stl_algobase.h:303:5: note: candidate: 'template<class _Tp, class _Compare> constexpr const _Tp& std::max(const _Tp&, const _Tp&, _Compare)'
  303 |     max(const _Tp& __a, const _Tp& __b, _Compare __comp)
      |     ^~~
/usr/include/c++/14/bits/stl_algobase.h:303:5: note:   candidate expects 3 arguments, 2 provided
In file included from /usr/include/c++/14/algorithm:61:
/usr/include/c++/14/bits/stl_algo.h:5706:5: note: candidate: 'template<class _Tp> constexpr _Tp std::max(initializer_list<_Tp>)'
 5706 |     max(initializer_list<_Tp> __l)
      |     ^~~
/usr/include/c++/14/bits/stl_algo.h:5706:5: note:   candidate expects 1 argument, 2 provided
/usr/include/c++/14/bits/stl_algo.h:5716:5: note: candidate: 'template<class _Tp, class _Compare> constexpr _Tp std::max(initializer_list<_Tp>, _Compare)'
 5716 |     max(initializer_list<_Tp> __l, _Compare __comp)
      |     ^~~
/usr/include/c++/14/bits/stl_algo.h:5716:5: note:   template argument deduction/substitution failed:
a.cc:53:58: note:   mismatched types 'std::initializer_list<_Tp>' and 'long long int'
   53 |                                                 ans = max(ans, board[ii][jj]);
      |                                                       ~~~^~~~~~~~~~~~~~~~~~~~