Add the Project Euler to the list of Challenge sites.
@@ -560,6 +560,7 @@ Challenge sites:
- [TopCoder](https://www.topcoder.com/)
- [Geeks for Geeks](https://practice.geeksforgeeks.org/explore/?page=1)
- [InterviewBit](https://www.interviewbit.com/)
+- [Project Euler](https://projecteuler.net/)
## The Study Plan
John Washam