Complete string permutations problem

Solves an InterviewCake.com problem that returns all of the permutations of a
string input. The problem states that it's acceptable to assume that your input
string will not have repeated characters, which is why using a Set is
acceptable. I like this solution because it builds a permutations tree and then
assembles all of the permutations by doing a DFT over that tree.

deepmind/recursive-string-permutations.py

@@ -0,0 +1,56 @@
+import unittest
+from itertools import permutations
+
+
+class Node(object):
+    def __init__(self, x):
+        self.value = x
+        self.children = []
+
+
+def make_tree(c, xs):
+    root = Node(c)
+    for x in xs:
+        root.children.append(make_tree(x, xs - {x}))
+    return root
+
+
+def get_permutations(xs):
+    xs = set(xs)
+    root = make_tree("", xs)
+    q, perms = [], set()
+    q.append(("", root))
+    while q:
+        c, node = q.pop()
+        if not node.children:
+            perms.add(c)
+        else:
+            for child in node.children:
+                q.append((c + child.value, child))
+    return perms
+
+
+# Tests
+class Test(unittest.TestCase):
+    def test_empty_string(self):
+        actual = get_permutations('')
+        expected = set([''])
+        self.assertEqual(actual, expected)
+
+    def test_one_character_string(self):
+        actual = get_permutations('a')
+        expected = set(['a'])
+        self.assertEqual(actual, expected)
+
+    def test_two_character_string(self):
+        actual = get_permutations('ab')
+        expected = set(['ab', 'ba'])
+        self.assertEqual(actual, expected)
+
+    def test_three_character_string(self):
+        actual = get_permutations('abc')
+        expected = set(['abc', 'acb', 'bac', 'bca', 'cab', 'cba'])
+        self.assertEqual(actual, expected)
+
+
+unittest.main(verbosity=2)