Consider the first sample test. We need to find a tree with 3 nodes that satisfies the following two constraints. The node labeled 2 with pre-order traversal should be in the left subtree of the node labeled 1 with pre-order traversal; the node labeled 3 with pre-order traversal should be in the right subtree of the node labeled 1. There is only one tree with three nodes that satisfies these constraints and its in-order traversal is (2, 1, 3).
Pre-order is the "root – left subtree – right subtree" order. In-order is the "left subtree – root – right subtree" order.
For other information regarding in-order and pre-order, see http://en.wikipedia.org/wiki/Tree_traversal.