title: saving Btrees to a disk file and read it
body: <p>I want to save a Btree(not sure a binary one) in a disk file.
  and then read it to the memory.
  some Level-order traversal may be a good way for a binary Btree.
  but if it is not a binary one.
  I build up the Btree from the leafnode to the rootnode in memory.
  I believe that I have to define some structures in the disk file and output the tree nodes.
  using some extra tag to identify a node in the file?
  how to traversal may be the key problem here.
  I coudn't figure out a good way to save the nodes and the pointers.
  and then read it. rconstruct the tree in memory.
  any good ideas?.
  thanks a lot.</p>