参考：

public void wallsAndGates(int[][] rooms) {
    for (int i = 0; i < rooms.length; i++)
        for (int j = 0; j < rooms[0].length; j++)
            if (rooms[i][j] == 0) dfs(rooms, i, j, 0);
}

private void dfs(int[][] rooms, int i, int j, int d) {
    if (i < 0 || i >= rooms.length || j < 0 || j >= rooms[0].length || rooms[i][j] < d) return;
    rooms[i][j] = d;
    dfs(rooms, i - 1, j, d + 1);
    dfs(rooms, i + 1, j, d + 1);
    dfs(rooms, i, j - 1, d + 1);
    dfs(rooms, i, j + 1, d + 1);
}