Project/Assignment 2 Solved

This assignment is intended to have you start working with stacks, queues, and linked lists.  There are several parts to the assignment, each described below.

For this assignment, you are provided with some starter code that defines the structures you'll be working with and prototypes the functions you'll be writing.  *It's important that you don't modify the function prototypes.*  To help grade your assignment, we will use a set of unit tests that assume these functions exist and have the same prototypes that are defined in the starter code.  If you change the prototypes, it will cause the unit tests to break, and your grade for the assignment will likely suffer.  Feel free, however, to write any additional functions you need to accomplish the tasks described below.

In this assignment, your work will be limited to the files `list_reverse.c`, `queue_from_stacks.c`, and `stack_from_queues.c`, where you will implement the functions described below. In addition to the descriptions below, there is thorough documentation in these files describing how each function should behave.

## 1. Implement a function to reverse a linked list in place

In `link.h`, a simple structure implementing a link in a singly-linked list is defined.  For this part of the assignment, you will implement a function called `list_reverse()` that takes as an argument a single link structure representing the head of a linked list, reverses that list, and returns the new head of the reversed list.  This function is prototyped in `list_reverse.h`, and you will implement it in `list_reverse.c`.  **Importantly, you must perform the list reversal in place and may not allocate any new memory in this function.**

## 2. Implement a queue using two stacks

In the files `stack.h` and `stack.c`, a simple stack data structure is implemented.  For the second part of this assignment, you will use two instances of this stack data structure to implement a queue.  In other words, you will implement a queue that uses two stacks to form the underlying container in which data is stored (instead of, for example, a dynamic array or a linked list).  For example, when the user calls `enqueue()` on your queue-from-stacks data structure, it will add the newly-enqueued element into one of the two stacks, as appropriate, and when the user calls `dequeue()`, your queue-from-stacks will remove the appropriate element from one of the two stacks.  To the user of your queue-from-stacks, it will behave just like a normal queue.

The interface of your queue-from-stacks is defined in `queue_from_stacks.h`, and you must complete each of the functions implementing the queue-from-stacks in `queue_from_stacks.c`.  Each of the functions in `queue_from_stacks.c` has a function header comment that describes more precisely how it should behave.

Importantly, you may only use the functions prototyped in `stack.h` to interface with your two stacks, and you may not access the underlying data directly.  Also, make sure your queue-from-stacks implementation does not have any memory leaks!

> Hint: think of one stack as an "inbox" and one stack as an "outbox".

## 3. Implement a stack using two queues

In the files `queue.h` and `queue.c`, a simple queue data structure is implemented.  For the third part of this assignment, you will use two instances of this queue data structure to implement a stack.  In other words, you will implement a stack that uses two queues to form the underlying container in which data is stored (instead of, for example, a dynamic array or a linked list).  For example, when the user calls `push()` on your stack-from-queues data structure, it will add the newly-pushed element into one of the two queues, as appropriate, and when the user calls `pop()`, your stack-from-queues will remove the appropriate element from one of the two queues.  To the user of your stack-from-queues, it will behave just like a normal stack.

The interface of your stack-from-queues is defined in `stack_from_queues.h`, and you must complete each of the functions implementing the stack-from-queues in `stack_from_queues.c`.  Each of the functions in `stack_from_queues.c` has a function header comment that describes more precisely how it should behave.

Importantly, you may only use the functions prototyped in `queue.h` to interface with your two queues, and you may not access the underlying data directly.  Also, make sure your stack-from-queues implementation does not have any memory leaks!

> Hint: there are two possible implementations of the stack-from-queues, one with an expensive pop operation and an efficient push operation, and one with an expensive push operation and an efficient pop operation.  Whichever of these you choose to implement, the key is knowing where in your two queues the most recently pushed element is.  This is always the next element to be popped from the stack.

## Testing your work

In addition to the starter code provided, you are also provided with some application code in `test.c` to help verify that your functions are behaving the way you want them to.  In particular, the code in `test.c` calls the functions you'll implement in this assignment, passing them appropriate arguments, and then prints the results.  You can use the provided `Makefile` to compile all of the code in the project together, and then you can run the testing code as follows:
```
make
./test
```
You can see some example output from a correct solution to the assignment in the file `example_output.txt`.

In order to verify that your memory freeing functions work correctly, it will be helpful to run the testing application through `valgrind`.

There is also a more extensive unit test suite for all of the functions you'll write for this assignment included in `unittest.c`.  After running `make`, you can run these unit tests as follows:
```
./unittest
```
There are several unit tests included, and if any unit test failures occur, an error message will be printed out indicating the line number in `unittest.c` from which the failure originated.  There are comments above each test that you can read to more thoroughly understand any given test failure.

## Submission

As always, we'll be using GitHub Classroom for this assignment, and you will submit your assignment via GitHub. Just make sure your completed files are committed and pushed by the assignment's deadline to the master branch of the GitHub repo that was created for you by GitHub Classroom. A good way to check whether your files are safely submitted is to look at the master branch of your assignment repo on the github.com website (i.e. http://github.com/OSU-CS261-F18/assignment-2-YourGitHubUsername/). If your changes show up there, you can consider your files submitted.

## Grading criteria

To grade your work, the TAs will use the tests in both `test.c` and `unittest.c` Your programs **MUST** compile and run on `flip.engr.oregonstate.edu`, so make sure you have tested your work there before you make your final submission, since a program that compiles and runs in one environment may not compile and run in another.  **Assignments that do not compile on flip will receive a grade of 0.**  If you do not have an ENGR account, you can create one at https://teach.engr.oregonstate.edu/.

The assignment is worth 100 total points, broken down as follows:

* 20 points: `list_reverse()` works as described above
* 40 points: all functions in your queue-from-stacks implementation correctly implement a queue
* 40 points: all functions in your stack-from-queues implementation correctly implement a stack

Note that we will only consider changes to `list_reverse.c`, `queue_from_stacks.c`, and `stack_from_queues.c` when grading your work.  Changes to other files will be ignored.