Add solution for challenge #40

Author: Mauricio Klein

README.md

@@ -39,3 +39,4 @@
 - [x] [Challenge 37: Active time](challenge-37/)
 - [x] [Challenge 38: Remove k-th Last Element From Linked List](challenge-38/)
 - [x] [Challenge 39: Remove Consecutive Nodes that Sum to 0](challenge-39/)
+- [x] [Challenge 40: Queue Using Two Stacks](challenge-40/)

challenge-40/Makefile

@@ -0,0 +1,4 @@
+test:
+	python test_*.py
+
+.PHONY: test

challenge-40/README.md

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+# Queue Using Two Stacks
+
+This problem was asked by Apple.
+
+## Description
+
+Implement a queue class using two stacks. A queue is a data structure that supports the FIFO protocol (First in = first out). Your class should support the enqueue and dequeue methods like a standard queue.
+
+## Example
+```python
+class Queue:
+  def __init__(self):
+    # Fill this in.
+    
+  def enqueue(self, val):
+    # Fill this in.
+
+  def dequeue(self):
+    # Fill this in.
+
+q = Queue()
+
+q.enqueue(1)
+q.enqueue(2)
+q.enqueue(3)
+
+print q.dequeue() # 1
+print q.dequeue() # 2
+print q.dequeue() # 3
+```

challenge-40/__init__.py

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+class Queue:
+  def __init__(self):
+    self.main_stack = []
+    self.aux_stack  = []
+   
+  #
+  # Time complexity: O(n) 
+  #
+  def enqueue(self, val):
+    # Move all elements from the main stack
+    # to the aux stack
+    while self.main_stack:
+      self.aux_stack.append(
+        self.main_stack.pop()
+      )
+
+    # Add the element to the inverted stack
+    self.aux_stack.append(val)
+
+    # Move elements back to the main stack
+    while self.aux_stack:
+      self.main_stack.append(
+        self.aux_stack.pop()
+      )
+
+  #
+  # Time complexity: O(1)
+  #
+  def dequeue(self):
+    return self.main_stack.pop()

challenge-40/solver.py

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+import unittest
+from solver import Queue
+
+class TestSolver(unittest.TestCase):
+  def test_queue(self):
+    q = Queue()
+
+    q.enqueue(1)
+    q.enqueue(2)
+    q.enqueue(3)
+
+    self.assertEqual(q.dequeue(), 1)
+    self.assertEqual(q.dequeue(), 2)
+    self.assertEqual(q.dequeue(), 3)
+
+if __name__ == "__main__":
+    unittest.main()