Change links to comply with author request

Ian Parberry, the author, requests that links point to a license page
leading to his books rather than directly to pdf. See
https://github.com/vhf/free-programming-books/issues/1769#issuecomment-1
70046046

free-programming-books.md

@@ -275,15 +275,14 @@ Original Source: [List of freely available programming books](http://web.archive
 * [Elementary Algorithms](https://github.com/liuxinyu95/AlgoXY) - Larry LIU Xinyu
 * [Foundations of Computer Science](http://infolab.stanford.edu/~ullman/focs.html) - Al Aho and Jeff Ullman
 * [Handbook of Graph Drawing and Visualization](https://cs.brown.edu/~rt/gdhandbook/)
-* [Lectures Notes on Algorithm Analysis and Computational Complexity (Fourth Edition)](http://larc.unt.edu/ian/books/free/lnoa.pdf) - University of North Texas (PDF)
+* [Lectures Notes on Algorithm Analysis and Computational Complexity (Fourth Edition)](https://larc.unt.edu/ian/books/free/license.html) - Ian Parberry (use form at bottom of license)
 * [LEDA: A Platform for Combinatorial and Geometric Computing](http://people.mpi-inf.mpg.de/~mehlhorn/LEDAbook.html)
 * [Linked List Basics](http://cslibrary.stanford.edu/103/LinkedListBasics.pdf) (PDF)
 * [Linked List Problems](http://cslibrary.stanford.edu/105/LinkedListProblems.pdf) (PDF)
 * [Matters Computational: Ideas, Algorithms, Source Code](http://www.jjj.de/fxt/fxtbook.pdf) (PDF)
 * [Open Data Structures: An Introduction](http://www.aupress.ca/index.php/books/120226) - Pat Morin
 * [Planning Algorithms](http://planning.cs.uiuc.edu)
-* [Problems on Algorithms (Second Edition)](http://larc.unt.edu/ian/books/free/poa.pdf) - University of North Texas (PDF)
-* [Purely Functional Data Structures](http://www.cs.cmu.edu/~rwh/theses/okasaki.pdf) (PDF)
+* [Problems on Algorithms (Second Edition)](https://larc.unt.edu/ian/books/free/license.html) - Ian Parberry (use form at bottom of license)* [Purely Functional Data Structures](http://www.cs.cmu.edu/~rwh/theses/okasaki.pdf) (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) (PDF)
 * [The Algorithm Design Manual](http://www8.cs.umu.se/kurser/TDBAfl/VT06/algorithms/BOOK/BOOK/BOOK.HTM)