Merge pull request #1452 from Akhilan/master

Adding materials of Graph Theory

free-programming-books.md

@@ -380,6 +380,7 @@ Original Source: [Free Programming books](http://stackoverflow.com/revisions/392
 * [Discrete Mathematics](http://home.iitk.ac.in/%7Earlal/book/mth202.pdf) (PDF) - Dr. Arbind K Lal, IIT Kanpur
 * [Elementary Differential Equations](http://ramanujan.math.trinity.edu/wtrench/texts/TRENCH_DIFF_EQNS_I.PDF) - William F. Trench
 * [Essentials of Metaheuristics](http://cs.gmu.edu/~sean/book/metaheuristics/) by Sean Luke
+* [Graph Theory](http://compalg.inf.elte.hu/~tony/Oktatas/TDK/FINAL/)
 * [Introduction to Probability](http://www.dartmouth.edu/~chance/teaching_aids/books_articles/probability_book/book.html) - Charles M. Grinstead and J. Laurie Snell
 * [Introduction to Probability and Statistics Spring 2014](http://ocw.mit.edu/courses/mathematics/18-05-introduction-to-probability-and-statistics-spring-2014/)
 * [Introduction to Statistical Thought](http://www.math.umass.edu/~lavine/Book/book.html) - Michael Lavine