Add 'universe/' from commit '8ad51b24dd8719840aac47134835ea25cfe1b0b8'

git-subtree-dir: universe
git-subtree-mainline: 15110e6de9f85537c7847267caa35fa068aea001
git-subtree-split: 8ad51b24dd8719840aac47134835ea25cfe1b0b8

1 files changed, 79 insertions, 0 deletions

diff --git a/universe/data_structures_and_algorithms/reverse-linked-list.py b/universe/data_structures_and_algorithms/reverse-linked-list.py
new file mode 100644
index 000000000000..b7396b20ce3f
--- /dev/null
+++ b/universe/data_structures_and_algorithms/reverse-linked-list.py
@@ -0,0 +1,79 @@
+import unittest
+
+
+################################################################################
+# Solution
+################################################################################
+# reverse :: List(a) -> List(a)
+def reverse(node):
+    curr = node
+    prev = None
+    while curr:
+        nxt = curr.next
+        curr.next = prev
+        prev = curr
+        curr = nxt
+    # Make sure to understand the spec! Debugging takes time. Rewriting takes
+    # time.
+    return prev
+
+
+################################################################################
+# Tests
+################################################################################
+class Test(unittest.TestCase):
+    class LinkedListNode(object):
+        def __init__(self, value, next=None):
+            self.value = value
+            self.next = next
+
+        def get_values(self):
+            node = self
+            values = []
+            while node is not None:
+                values.append(node.value)
+                node = node.next
+            return values
+
+    def test_short_linked_list(self):
+        second = Test.LinkedListNode(2)
+        first = Test.LinkedListNode(1, second)
+
+        result = reverse(first)
+        self.assertIsNotNone(result)
+
+        actual = result.get_values()
+        expected = [2, 1]
+        self.assertEqual(actual, expected)
+
+    def test_long_linked_list(self):
+        sixth = Test.LinkedListNode(6)
+        fifth = Test.LinkedListNode(5, sixth)
+        fourth = Test.LinkedListNode(4, fifth)
+        third = Test.LinkedListNode(3, fourth)
+        second = Test.LinkedListNode(2, third)
+        first = Test.LinkedListNode(1, second)
+
+        result = reverse(first)
+        self.assertIsNotNone(result)
+
+        actual = result.get_values()
+        expected = [6, 5, 4, 3, 2, 1]
+        self.assertEqual(actual, expected)
+
+    def test_one_element_linked_list(self):
+        first = Test.LinkedListNode(1)
+
+        result = reverse(first)
+        self.assertIsNotNone(result)
+
+        actual = result.get_values()
+        expected = [1]
+        self.assertEqual(actual, expected)
+
+    def test_empty_linked_list(self):
+        result = reverse(None)
+        self.assertIsNone(result)
+
+
+unittest.main(verbosity=2)