# Lab 11 — Stacks and Queues

In this lab, you will implement queues in different ways, and then fix memory leaks in the provided program. Start by downloading the provided program [levelOrder.cpp](levelOrder.cpp). The provided program [levelOrder.cpp](levelOrder.cpp) traverses a binary tree by level order. It prints the following message to STDOUT:

```console
Level Order Traversal: 1 2 3 4 5 6 7
Level Order Traversal: 1 2 3 4 5 6 7 8 9
Level Order Traversal: 1 2 3 4 5 6 7 8
```


## Checkpoint 1:

*estimate: 30-40 minutes*

First, read the code of the provided program, and run the program to see its output. 

- Play this [animation](https://jidongxiao.github.io/CSCI1200-DataStructures/animations/trees/level_order/index.html) to see how level order traverse works.

Then, replace the STL queue library with the STL stack library, and implement the queue using two stacks. Do not change the *main* function. Do not change the *levelOrderTraversal* function, except this line:

```cpp
std::queue<TreeNode*> myQueue;
```

**To complete this checkpoint**: Show a TA your program, and your test results. Your program should still produce the same results as the original program. And you must be able to explain your program.

## Checkpoint 2:

*estimate: 30-40 minutes*

Re-implement the queue using the STL list library. Still, do not change the *main* function, and do not change the *levelOrderTraversal* function, except this line:

```cpp
std::queue<TreeNode*> myQueue;
```

**To complete this checkpoint**: Show a TA your program, and your test results. Your program should still produce the same results as the original program. And you must
 be able to explain your program.

## Checkpoint 3:

*estimate: 15-20 minutes*

The provided program clearly has memory leaks. Fix the memory leaks.

**To complete this checkpoint**: Show a TA your program, and your test results with either Valgrind or DrMemory.