Determine if a binary tree is subtree of another binary tree using pre-order and in-order strings

Very interesting question. You seem to be correct. I suppose the issue that you mention arises due to different definitions of subtree in math (graph theory) and computer science. In graph theory T2 is a proper subtree of T1.

Assuming you got this from the book "Cracking the Coding Interview", the author also mentions that to distinguish between nodes with the same values, one should also print out the null values.

This would also solve your problem with the definition of a subtree (which is correct as also described in the book)

preorder T2: "1, 2, null, null, 3, null, null" preorder T1: "1, 2, null, null, 3, null, 4, null, null" inorder T2: "null, 2, null, 1, null, 3, null" inorder T1: "null, 2, null, 1, null, 3, null, 4, null"

As you can see, T2 is not a subtree of T1