1 files changed, 66 insertions, 0 deletions
diff --git a/scratch/data_structures_and_algorithms/queue-two-stacks.py b/scratch/data_structures_and_algorithms/queue-two-stacks.py
new file mode 100644
index 000000000000..63da08ebf79a
--- /dev/null
+++ b/scratch/data_structures_and_algorithms/queue-two-stacks.py
@@ -0,0 +1,66 @@
+import unittest
+
+
+################################################################################
+# Solution
+################################################################################
+class QueueTwoStacks(object):
+    def __init__(self):
+        self.a = []
+        self.b = []
+
+    def enqueue(self, x):
+        self.a.append(x)
+
+    def dequeue(self):
+        if self.b:
+            return self.b.pop()
+        else:
+            while self.a:
+                self.b.append(self.a.pop())
+            return self.dequeue()
+
+
+################################################################################
+# Tests
+################################################################################
+class Test(unittest.TestCase):
+    def test_basic_queue_operations(self):
+        queue = QueueTwoStacks()
+        queue.enqueue(1)
+        queue.enqueue(2)
+        queue.enqueue(3)
+        actual = queue.dequeue()
+        expected = 1
+        self.assertEqual(actual, expected)
+        actual = queue.dequeue()
+        expected = 2
+        self.assertEqual(actual, expected)
+        queue.enqueue(4)
+        actual = queue.dequeue()
+        expected = 3
+        self.assertEqual(actual, expected)
+        actual = queue.dequeue()
+        expected = 4
+        self.assertEqual(actual, expected)
+
+    def test_error_when_dequeue_from_new_queue(self):
+        queue = QueueTwoStacks()
+        with self.assertRaises(Exception):
+            queue.dequeue()
+
+    def test_error_when_dequeue_from_empty_queue(self):
+        queue = QueueTwoStacks()
+        queue.enqueue(1)
+        queue.enqueue(2)
+        actual = queue.dequeue()
+        expected = 1
+        self.assertEqual(actual, expected)
+        actual = queue.dequeue()
+        expected = 2
+        self.assertEqual(actual, expected)
+        with self.assertRaises(Exception):
+            queue.dequeue()
+
+
+unittest.main(verbosity=2)