Added lookup to BST

BST.py

@@ -55,6 +55,15 @@ class BST:
                 parent.left = to_add
             self.size += 1
 
+    def lookup(self, value):
+        current = self.root
+        while current.value != value and current is not None:
+            if value < current.value:
+                current = current.left
+            elif value > current.value:
+                current = current.right
+        return current
+
     def in_order(self):
         ret = []
         in_order_rec(self.root, ret)