[REFACTOR] clean code: variable naming

Author: Gonzalo Diaz

src/hackerrank/lib/tree.py

@@ -21,8 +21,8 @@ class BinarySearchTree:
     def __init__(self, nodes: list[int] | None = None):
         self.root = None
         if nodes is not None:
-            for x in nodes:
-                self.create(x)
+            for node in nodes:
+                self.create(node)
 
     def create(self, val):
         if self.root is None: