Added small clarification to postfix and prefix notation in tree-recipes

Author: TianLangHin

competition/tree-recipes/prob.md

@@ -29,6 +29,13 @@ Division A wants to receive these recipes in
 while Division B wants to receive these recipes in
 [*prefix*](https://en.wikipedia.org/wiki/Tree_traversal#Pre-order,_NLR) notation.
 
+To clarify, let us consider one tree node named `A`,
+that has two child nodes: `B` and `C`.
+**Postfix** notation writes the contents of all child nodes before the node itself,
+hence `B C A` would be the postfix notation of the structure of node `A`.
+Meanwhile, **prefix** notation writes the node first, before the child nodes.
+Hence, `A B C` would be the prefix notation of the structure of node `A`.
+
 The format of these trees is as follows:
 Every node in the tree is named by either **three letters** or a **number**.
 A word (e.g., `abc`) indicates a leaf node, while