  The motion of charmonium in heavy ion collisions is described by a three
dimensional transport equation with initial production and continuous
regeneration in hot medium. The observation of apparently stronger $J/\psi$
suppression at forward rapidity compared to that at midrapidity, so called
$J/\psi$ puzzle at RHIC, can well be explained by the competition between the
two production mechanisms. At LHC, however, the rapidity dependence of the
$J/\psi$ production is dominated by the regeneration process.
