Add Problem Solving with Algorithms and Data Struc

Add Problem Solving with Algorithms and Data Structures. Since it is
written in python, I have added it twice.

This was suggested by Fabian Dablander on the Python community on
Facebook.

free-programming-books.md

@@ -141,6 +141,7 @@
 * [Algorithms for Programmers: Ideas and Source Code](http://www.jjj.de/fxt/fxtbook.pdf)
 * [Sequential and parallel sorting algorithms](http://www.inf.fh-flensburg.de/lang/algorithmen/sortieren/algoen.htm)
 * [Text Algorithms](http://igm.univ-mlv.fr/~mac/REC/text-algorithms.pdf)
+* [Problem Solving with Algorithms and Data Structures](http://interactivepython.org/courselib/static/pythonds/index.html)
 
 ####Theoretical Computer Science
 * [Structure and Interpretation of Computer Programs](http://mitpress.mit.edu/sicp/)
@@ -740,6 +741,7 @@
 * [Python Scientific Lecture Notes](http://scipy-lectures.github.io/)
 * [Snake Wrangling For Kids](http://www.briggs.net.nz/snake-wrangling-for-kids.html)
 * [Think Python](http://www.greenteapress.com/thinkpython/) - Allen B. Downey
+* [Problem Solving with Algorithms and Data Structures](http://interactivepython.org/courselib/static/pythonds/index.html)
 
 ####Django