Add recursive example for finding a key in nested boxes (Python) (#290)

03_recursion/python/08_look_for_key.py

@@ -0,0 +1,50 @@
+# Item have 2 types: key or box
+class Item:
+    def __init__(self, is_key=False):
+        self.is_key = is_key 
+
+        # If not a key, it's a box that can hold items 
+        self.items_in_box = [] 
+
+    def is_a_box(self):
+        return not self.is_key
+
+    def is_a_key(self):
+        return self.is_key
+
+
+def look_for_key(box: Item):
+    for item in box.items_in_box:
+        if item.is_a_box():
+            # recursive case
+            look_for_key(item)
+        elif item.is_a_key():
+            # base case
+            print("found the key!") 
+
+
+"""
+main_box
+├── box_A
+│   ├── box_B
+│   └── box_C
+└── box_D
+    └── box_E
+        └── key
+"""
+main_box = Item()
+
+box_A = Item()
+box_B = Item()
+box_C = Item()
+box_A.items_in_box = [box_B, box_C]
+
+box_D = Item()
+box_E = Item()
+key = Item(is_key = True)
+box_E.items_in_box = [key]
+box_D.items_in_box = [box_E]
+
+main_box.items_in_box = [box_A, box_D]
+
+look_for_key(main_box)