added stacks queues initial draft

Author: iriswang

stacks_queues/figures/queues.png

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+## Introduction
+
+Stacks and queues are foundational data structures that are useful when adding and removing in particular orders. It's important to be comfortable with these two data structures.
+
+## Stacks
+A **stack** is a data structure that stores objects in which the most recently stored objects are the first ones to be removed, (LIFO: last in, first out). An example to help you remember the mechanics of a stack is to associate it with stacks in real life. With a stack of plates, the plates that are placed on top of a stack will be the first ones that are removed from the top!
+
+![](https://i.imgur.com/qMSmxsa.png)
+
+It's important to know the common operations of a stack. The two key stack operations are:
+1) pop(): removing an item from the stack in a last in, first out order (LIFO)
+2) push(item): adding an item to the stack
+
+
+## Queues
+A **queue** is a data structure that stores objects in which the most stored objects are the first ones to be removed. A helpful acronym associated with queues is FIFO, first in first out. An example to help you remember the mechanics of a queue is to associate it with queues in real life. With a queue of people waiting to get a seat in a restaurant, the first people to get in the queue will be the first people seated at that restaurant.
+
+![](https://i.imgur.com/NKuZd0s.png)
+
+It's important to know the common operations associated with a queue. The two important queue operations are:
+1) dequeue(): removing an item from the queue in a first in, first out order (FIFO)
+2) enqueue(item): adding an item to the queue
+
+## Key takeaways
+* Stacks are very useful for it's backtracking features. For example, parsing questions tend to use stacks because of the LIFO property.
+* Stacks can be used to implement recursive solutions iteratively.
+* Queues are useful when the ordering of the data matters as it preserves that ordering. For example, they're used for caching.
+
+## Example problems
+* Stacks: 
+  * [Implement a queue with stacks](https://leetcode.com/problems/implement-queue-using-stacks/description/)
+
+* Queues:
+  * [LRU Cache](https://leetcode.com/problems/lru-cache/)
+
+## Resources
+### Guides
+* [Overview of stacks & queues with applications](https://www.cs.cmu.edu/~adamchik/15-121/lectures/Stacks%20and%20Queues/Stacks%20and%20Queues.html)
+* [In depth stacks guide](https://medium.com/basecs/stacks-and-overflows-dbcf7854dc67)
+* [In depth queues guide](https://medium.com/basecs/to-queue-or-not-to-queue-2653bcde5b04)
+
+### Libraries
+* [Java queue library](https://docs.oracle.com/javase/7/docs/api/java/util/Queue.html)
+* [Java stack library](
+https://docs.oracle.com/javase/7/docs/api/java/util/Stack.html)
+* [Python queue library](
+https://docs.python.org/2/tutorial/datastructures.html#using-lists-as-queues)
+* [Python stack library](https://docs.python.org/2/tutorial/datastructures.html#using-lists-as-stacks)
+