RosettaCodeData/Task/Tree-from-nesting-levels/J/tree-from-nesting-levels-4.j

   t=: ;:'(a b c) d (e f g)'
   p=: ;:'()'
   d=: +/\-/p=/t
   k=: =/p=/t
   merge d <@]^:[&.>&(k&#) t
┌───────┬─┬───────┐
│┌─┬─┬─┐│d│┌─┬─┬─┐│
││a│b│c││ ││e│f│g││
│└─┴─┴─┘│ │└─┴─┴─┘│
└───────┴─┴───────┘