// k4 map

n:!10                           / nodes
t:n!0 0 0 2 2 2 4 4 7 7         / tree
l:~n in t                       / leaves
d:n!l*(10?100)%l                / data (NaN at non-leaves)
f:50<                           / predicate
N@:<N:?,/t\'&l&f d              / subtree nodes
T:N#t                           / subtree
D:N#d                           / subtree data

/ pretty print 
pp:{-1{(x#""),($z)," ",$y}'[#:'x\'!x;y@!y;!x];}

\

 pp[T;D]
 0
  1 67
  2
   3 73
   4
    6 53
    7
     8 83