System's Twilight Hint Guide:

General Hints for Solving Trees

1. It's helpful to note that it's completely impossible to reach certain nodes from certain other nodes. For instance, on the first tree, you can already see that the goal can only be reached from certain nodes:
   This cuts down on the number of nodes you have to consider by a lot.
2. It's also helpful to figure out which paths are accessible and which paths are not. On some of the harder levels, you may have to do this by trial and error.