Merge pull request #1827 from kiriappeee/master

Added advent of code. Closes #1773

free-programming-books.md

@@ -1541,7 +1541,7 @@ For resources on Angular, Backbone, D3, Dojo, Ember, Express, jQuery, Knockout,
 ### Objective-C
 
 * [Google's Objective-C Style Guide](http://google-styleguide.googlecode.com/svn/trunk/objcguide.xml)
-* [Object-Oriented Programming with Objective-C](https://developer.apple.com/library/mac/documentation/Cocoa/Conceptual/OOP_ObjC/OOP_ObjC.pdf) (PDF)
+* [Object-Oriented Programming with Objective-C](https://developer.apple.com/library/ios/documentation/Cocoa/Conceptual/OOP_ObjC/Introduction/Introduction.html#//apple_ref/doc/uid/TP40005149)
 * [Objective-C Succinctly, Syncfusion](https://www.syncfusion.com/resources/techportal/ebooks/objective-c) (PDF, Kindle) *(Just fill the fields with any values)*
 * [Programming With Objective-C](https://developer.apple.com/library/mac/documentation/Cocoa/Conceptual/ProgrammingWithObjectiveC/Introduction/Introduction.html)
 * [Try Objective-C Book](http://tryobjectivec.codeschool.com)

problem-sets-competitive-programming.md

@@ -26,6 +26,7 @@
 
 ### Problem Sets
 
+* [Advent Of Code](http://adventofcode.com)
 * [Anarchy Golf](http://golf.shinh.org)
 * [CareerCup](http://www.careercup.com)
 * [CheckIO](http://www.checkio.org)