comefrom: <doocs>0430.Flatten a Multilevel Doubly Linked List_1.py
code: def preorder(pre, cur):
      if cur is None:
          return pre
      cur.prev = pre
      pre.next = cur
      
      t = cur.next
      tail = preorder(cur, cur.child)
      cur.child = None
      return preorder(tail, t)
masked: def sample_funcname ( var_1 , var_2 ) :
      if var_2 is None :
          return var_1
      var_2.prev = var_1
      var_1.next = var_2
      var_6 = var_2.next
      var_8 = var_0 ( var_2 , var_2.child )
      var_2.child = None
      return var_0 ( var_8 , var_6 )
  