def find_empty_cells(gr):
    """
    Collects all the (i,j) free couples of the grid 
    :param gr: 2D list
    :return: list
    """
    l = list()
    for i in range(0,9):
        for j in range(0,9):
            if(gr[i][j] == 0):
                l.append([i, j])
    return l