This app was mentioned in 3 comments, with an average of 5.67 upvotes
Download the app GeeksForGeeks on Android Play store. Enjoy the problems.
https://play.google.com/store/apps/details?id=free.programming.programming&hl=en
Learn Data Structure and Algorithm from this website :
http://www.geeksforgeeks.org/ .
They also have a mobile app as well.
https://play.google.com/store/apps/details?id=free.programming.programming
Once you have done that. Head over to LeetCode / HackerRank and solve all programming questions.
Do this for one year. Then you will be fit for Google internship.
Lot of problems mentioned here with solutions. http://www.techiedelight.com/
If you need a mobile app then I recommend GeeksforGeeks app. https://play.google.com/store/apps/details?id=free.programming.programming&hl=en
Their website is also good .