Add notes on stacks

stacks.md

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+stacks
+======
+
+A **stack** mimics a stack of paper.
+You add and remove things from the top.
+This is also known as "Last In, First Out" (LIFO).
+
+## Requirements
+
+- view top of the stack: `top()` or `peek()`
+- remove top of the stack: `pop()`
+  - this may or may not return the top item
+- add to the top of the stack: `push(item)`
+
+Note that you shouldn't need to (or be able to) iterate through a stack.
+Knowing the size of the stack may also be optional.
+
+## Conceptualization
+
+- Keep track of the head, or top of the stack
+
+## Implementation
+
+A list can sometimes be used to mimic a stack:
+- for a Python list:
+  - `list.append()`
+  - `list.pop()`
+  - `list[-1]` to access top
+- for a JavaScript array:
+  - `array.push()`
+  - `array.pop()`
+  - `array[array.length - 1]` to access top
+
+Other language libraries use another ADT to implement a stack:
+- C++ `stack` (uses a `deque`)
+- Java's `Stack` (uses a `vector`)
+
+Since lists can be used to implement a stack, it follows that they can also be implemented directly using list implementation techniques.
+
+## Applications
+
+Reversing a string: 
+- iterate through a string pushing each letter onto stack
+- pop off all elements into a new string
+
+Stack machines, like an interpreter for Reverse Polish Notation.
+
+The _shunting-yard algorithm_ parses infix expression (1 + 2) to produce postfix ones (RPN) or an AST. Uses both stack and queue.
+
+TODO: Backtracking
+
+Compilers & Interpreters:
+- space for parameters, local variables, and return address of each function call (_stack frame_)
+- compiler's syntax check for matching braces
+
+Virtual (stack) machines:
+- JVM (Java, Clojure, Scala, Groovy)
+- CLR & Mono (C#)
+- CPython (Python)
+- Ruby MRI (>=1.9)
+
+## REFERENCES
+
+[`Class Stack<E>`](https://docs.oracle.com/javase/7/docs/api/java/util/Stack.html). Java Collections stack.
+
+[`stack<T, Sequence>`](http://www.sgi.com/tech/stl/stack.html). C++ STL stack.
+
+[Using Lists as Stacks](https://docs.python.org/2/tutorial/datastructures.html#using-lists-as-stacks). Data Structures. Python.
+
+[Array](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array). Javascript reference. MDN.