[2,3,1], [3,2,1] is the input. Maybe I'm just getting it all wrong, but if [3,2,1] is the post order traversal, that means 1 is the root. Since 1 is also the last element of inorder traversal, that means there is no right subtree. This makes 3 the left most node, which means it must be visited first in the inorder traversal. So, I'm not quite sure how [2,3,1], [3,2,1] can be a valid input?