As Far from Land as Possible — C++

// Time:  O(m * n)
// Space: O(m * n)

class Solution {
public:
    int maxDistance(vector<vector<int>>& grid) {
        static const vector<pair<int, int>> directions{{0, 1}, {1, 0}, {0, -1}, {-1, 0}};
        queue<pair<int, int>> q;
        for (int i = 0; i < grid.size(); ++i) {
            for (int j = 0; j < grid[i].size(); ++j) {
                if (grid[i][j]) {
                    q.emplace(i, j);
                }
            }
        }
        if (q.size() == grid.size() * grid[0].size()) {
            return -1;
        }
        int level = -1;
        while (!q.empty()) {
            queue<pair<int, int>> next_q;
            while (!q.empty()) {
                const auto [x, y] = q.front(); q.pop();
                for (const auto& [dx, dy] : directions) {
                    const auto& nx = x + dx;
                    const auto& ny = y + dy;
                    if (!(0 <= nx && nx < grid.size() && 
                          0 <= ny && ny < grid[0].size() && 
                          grid[nx][ny] == 0)) {
                        continue;
                    }
                    next_q.emplace(nx, ny);
                    grid[nx][ny] = 1;
                }
            }
            q = move(next_q);
            ++level;
        }
        return level;
    }
};