Update is_sorted.py (keon#402)

* Update is_sorted.py

1)The first 2 return statements have been changed to break statements. This ensures that the code after the first for loop is executed. Previously it would never execute the latter part.
2)The second append statement should add elements to stack and not storage_stack. Try the test case [3,4,7,8,5,6]. The program returns True. It should return False.

* Add test for is_sorted

algorithms/stack/is_sorted.py

@@ -13,15 +13,15 @@ def is_sorted(stack):
     storage_stack = []
     for i in range(len(stack)):
         if len(stack) == 0:
-            return True
+            break
         first_val = stack.pop()
         if len(stack) == 0:
-            return True
+            break
         second_val = stack.pop()
         if first_val < second_val:
             return False
         storage_stack.append(first_val)
-        storage_stack.append(second_val)
+        stack.append(second_val)
 
     # Backup stack
     for i in range(len(storage_stack)):

tests/test_stack.py

@@ -25,6 +25,7 @@ def test_is_sorted(self):
         # Test case: bottom [6, 3, 5, 1, 2, 4] top
         self.assertFalse(is_sorted([6, 3, 5, 1, 2, 4]))
         self.assertTrue(is_sorted([1, 2, 3, 4, 5, 6]))
+        self.assertFalse(is_sorted([3, 4, 7, 8, 5, 6]))
 
     def test_remove_min(self):
         # Test case: bottom [2, 8, 3, -6, 7, 3] top