add princeton algorithms

to close #2491

free-courses-en.md

@@ -50,6 +50,8 @@
 * [Berkeley University CS 61B: Data Structures](http://datastructur.es/sp16/)
 * [IIT Bombay Foundation of Data Structures (CS213.1x)](https://courses.edx.org/courses/course-v1:IITBombayX+CS213.1x+1T2017/course/)
 * [Introduction to Algorithms (SMA 5503)](https://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-046j-introduction-to-algorithms-sma-5503-fall-2005) - Charles Leiserson
+* [Princeton University Algorithms, Part 1](https://www.coursera.org/learn/algorithms-part1)
+* [Princeton University Algorithms, Part 2](https://www.coursera.org/learn/algorithms-part2)
 * [Stanford University Algorithms: Design and Analysis, Part 1](http://online.stanford.edu/course/algorithms-design-and-analysis-part-1)
 * [Stanford University Algorithms: Design and Analysis, Part 2](http://online.stanford.edu/course/algorithms-design-and-analysis-part-2)