Added 2 new sources in problem-sets-competitive-programming.md (#10556)

* Update problem-sets-competitive-programming.md

Added 2 new sites for competitive programming  problem sets.

* create ladders and add appropriate links

* Update problem-sets-competitive-programming.md

more/problem-sets-competitive-programming.md

@@ -4,6 +4,7 @@
 * [CTF Capture the Flag](#capture-the-flag)
 * [Data science](#data-science)
 * [HTML and CSS](#html-and-css)
+* [Ladders](#ladders)
 * [Problem Sets](#problem-sets)
 
 
@@ -44,7 +45,6 @@
 * [oj.uz](https://oj.uz)
 * [Sigmageek](https://sigmageek.com)
 * [Sphere Online Judge](http://www.spoj.com/contests)
-* [Striver’s CP Sheet](https://takeuforward.org/interview-experience/strivers-cp-sheet/)
 * [Techgig](https://www.techgig.com)
 * [Topcoder](https://www.topcoder.com)
 * [Toph](https://toph.co)
@@ -82,10 +82,18 @@
 * [Frontend Mentor](https://www.frontendmentor.io/challenges?languages=CSS&sort=difficulty%7Casc&type=free) *(email address requested)*
 
 
+### Ladders
+
+* [A2 Online Judge](https://a2oj.netlify.app)
+* [Atcoder Problems](https://kenkoooo.com/atcoder/#/table)
+* [C2 Ladders](https://c2-ladders-juol.onrender.com)
+* [Striver's A2Z DSA Course/sheet](https://takeuforward.org/strivers-a2z-dsa-course/strivers-a2z-dsa-course-sheet-2)
+* [Striver’s CP Sheet](https://takeuforward.org/interview-experience/strivers-cp-sheet/)
+
+
 ### Problem Sets
 
 * [500 Data structures and algorithms interview questions and their solutions in C++](https://www.quora.com/q/techiedelight/500-Data-Structures-and-Algorithms-interview-questions-and-their-solutions)
-* [A2 Online Judge](https://a2oj.netlify.app)
 * [Advent Of Code](http://adventofcode.com)
 * [AdventJS - 25 días de retos con JavaScript](https://adventjs.dev) - Miguel Ángel Durán «midudev» *(GitHub account requested, not required)*
 * [Anarchy Golf](http://golf.shinh.org)
@@ -125,7 +133,6 @@
 * [Python Practice Projects](http://pythonpracticeprojects.com)
 * [Rosalind](http://rosalind.info/problems/locations/)
 * [Sphere Online Judge](http://www.spoj.com/problems/classical)
-* [Striver's A2Z DSA Course/sheet](https://takeuforward.org/strivers-a2z-dsa-course/strivers-a2z-dsa-course-sheet-2)
 * [TalentBuddy](http://www.talentbuddy.co/blog/)
 * [The Ultimate Topic List(with Tutorials, Problems, and Templates)](https://blog.shahjalalshohag.com/topic-list/) - Shahjalal Shohag
 * [Timus Online Judge](http://acm.timus.ru)