  We present an algorithm determining where to relocate objects inside a
cluttered and confined space while rearranging objects to retrieve a target
object. Although methods that decide what to remove have been proposed,
planning for the placement of removed objects inside a workspace has not
received much attention. Rather, removed objects are often placed outside the
workspace, which incurs additional laborious work (e.g., motion planning and
execution of the manipulator and the mobile base, perception of other areas).
Some other methods manipulate objects only inside the workspace but without a
principle so the rearrangement becomes inefficient.
  In this work, we consider both monotone (each object is moved only once) and
non-monotone arrangement problems which have shown to be NP-hard. Once the
sequence of objects to be relocated is given by any existing algorithm, our
method aims to minimize the number of pick-and-place actions to place the
objects until the target becomes accessible. From extensive experiments, we
show that our method reduces the number of pick-and-place actions and the total
execution time (the reduction is up to 23.1% and 28.1% respectively) compared
to baseline methods while achieving higher success rates.
