add CTFtime to problem-sets-competitive-programming.md (#3468)

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problem-sets-competitive-programming.md

@@ -2,6 +2,7 @@
 
 * [Competitive Programming](#competitive-programming)
 * [Data science](#data-science)
+* [Information security](#information-security)
 * [Problem Sets](#problem-sets)
 
 
@@ -36,6 +37,11 @@
 * [Kaggle](https://www.kaggle.com)
 
 
+### Information security
+
+* [CTFtime](https://ctftime.org)
+
+
 ### Problem Sets
 
 * [500 Data structures and algorithms interview questions and their solutions in C++](https://techiedelight.quora.com/500-Data-structures-and-algorithms-interview-questions-and-their-solutions-in-C%2B%2B)