Pre-order + post-order to in-order
Perl, 69 66 62 56 53 bytes

Includes +1 for -p

Takes postorder followed by preorder as one line separated by space on STDIN (notice the order of pre and post). Nodes are represented as unique letters (any character that is not space or newline is OK).

inpost.pl &lt;&lt;&lt; "98231 12983"


inpost.pl:

#!/usr/bin/perl -p
s%(.)(.)+\K(.)(.+)\3(\1.*)\2%$4$5$3$2%&amp;&amp;redo;s;.+ ;;


Using the original purely numeric format needs a lot more care to exactly identify a single number and comes in at 73 bytes

#!/usr/bin/perl -p
s%\b(\d+)(,\d+)+\K,(\d+\b)(.+)\b\3,(\1\b.*)\2\b%$4$5,$3$2%&amp;&amp;redo;s;.+ ;;


Use as

inpostnum.pl &lt;&lt;&lt; "11,12,10,2,8,4,5,3,7 7,8,10,11,12,2,3,4,5"