Walls and Gates 1

class Solution {
public:
    void wallsAndGates(vector<vector<int>>& rooms) {
        if (rooms.empty()) return;
        int m = rooms.size(), n = rooms[0].size();
        for (int i = 0; i < m; i++) {
            for (int j = 0; j < n; j++) {
                if (!rooms[i][j]) {
                    queue<Grid> grids;
                    grids.push(Grid(i + 1, j, 1));
                    grids.push(Grid(i - 1, j, 1));
                    grids.push(Grid(i, j + 1, 1));
                    grids.push(Grid(i, j - 1, 1));
                    while (!grids.empty()) {
                        Grid grid = grids.front();
                        grids.pop();
                        int r = grid.r, c = grid.c, d = grid.d;
                        if (r < 0 || r >= m || c < 0 || c >= n || rooms[r][c] < d)
                            continue;
                        rooms[r][c] = d;
                        grids.push(Grid(r + 1, c, d + 1));
                        grids.push(Grid(r - 1, c, d + 1));
                        grids.push(Grid(r, c + 1, d + 1));
                        grids.push(Grid(r, c - 1, d + 1));
                    }
                }
            }
        }
    }
private:
    struct Grid {
        int r, c, d;
        Grid(int _r, int _c, int _d) : r(_r), c(_c), d(_d) {}
    };
};