Is there an app for coding challenges like that on hackerrank website ?

Answer 1

There is a lot. Some time ago we listed a lot of these websites.

1. Codeforces
2. Topcoder
3. Hackerrank
4. Code chef
5. Spoj
Many more...

My favorite is codeforces. There are two levels of difficulty - div2 is easier, div1 is much harder.

Answer 2

Apps don't have the level of refinement, documentations of challenges and terms related to it so this time the web wins. Try visiting reddit.com/r/dailyprogrammer