def optStrajAdvHelp(strajDists, strajCov, r, c3):
    """ Computes subtrajectory of a trajectory with maximum coverage cost ratio
        Args:
            strajDists: list having subtraj-distance pairs of a trajectory.
            strajCov: dict storing points of subtrajs.
            r (float): guess coverage-cost ratio.
            c3 : parameter of greedy algorithm.

        Returns:
            (subtraj, float) or (None,None).
    """
    temp, stra, dista = 0, None, None
    for i in range(len(strajDists)):
        straj, dist = strajDists[i][0], strajDists[i][1]
        cov = strajCov[straj]
        if temp < cov - c3 * r * dist:
            temp = cov - c3 * r * dist
            stra, dista = straj, dist

    return (stra, dista)