Add InterviewCake.com examples

Adds some of the code I generated while studying for a role transfer at Google
using the fantastic resource, InterviewCake.com. This work predates the
mono-repo.

I should think of ways to DRY up this code and the code in
crack_the_coding_interview, but I'm afraid I'm creating unnecessary work for
myself that way.

data_structures_and_algorithms/permutations.py

@@ -0,0 +1,55 @@
+class Node(object):
+    # ctor :: a -> [a] -> Node(a)
+    def __init__(self, value, children=[]):
+        self.value = value
+        self.children = children
+
+
+# is_leaf :: Node(a) -> Boolean
+def is_leaf(node):
+    return len(node.children) == 0
+
+
+# enumerate :: Node(a) -> Set(List(a))
+def enumerate(node):
+    current = []
+    result = []
+    q = []
+
+    q.append(node)
+
+    while q:
+        x = q.pop()
+        print(x.value)
+
+        for c in x.children:
+            q.append(c)
+
+        current.append(x.value)
+        print(current)
+
+        if is_leaf(x):
+            result.append(current)
+            print("Reseting current")
+            current = []
+
+    return result
+
+
+node = Node('root', [
+    Node('a', [
+        Node('b', [Node('c')]),
+        Node('c', [Node('b')]),
+    ]),
+    Node('b', [
+        Node('a', [Node('c')]),
+        Node('c', [Node('a')]),
+    ]),
+    Node('c', [
+        Node('a', [Node('b')]),
+        Node('b', [Node('a')]),
+    ])
+])
+
+print('----------')
+print(enumerate(node))