The most common focus area for Software Developer Intern position in any big silicon valley company are Data Structure, Algorithms, Complexity, System Design, Multi-Threading. 

The best preparation materials:-
List of useful books

List of site to improve coding skills



Tip:- 
Solve all the problems on white board or on blank paper.
 



Other Materials-
  • This one has good examples of problem with solution for each topic
  • To understand algorithms and data structure visually
  • For system design