Add coding exercises for Facebook interviews

Add attempts at solving coding problems to Briefcase.

scratch/facebook/linked-list-cycles.py

@@ -0,0 +1,26 @@
+import random
+
+from linked_list import Node
+
+def contains_cycle(node):
+    one = node
+    two = node
+    while two.next and two.next.next:
+        one = one.next
+        two = two.next.next
+        if one == two:
+            return True
+    return False
+
+xs = Node(1, Node(2, Node(3)))
+assert not contains_cycle(xs)
+print("Success!")
+
+a = Node(1)
+b = Node(2)
+c = Node(3)
+a.next = b
+b.next = c
+c.next = random.choice([a, b, c])
+assert contains_cycle(a)
+print("Success!")