NVIDIA Software Engineer Student interview questions

write a data structure that you can implement these function in the most efficient way: getVal(int index) setVal(int index, char val) setAllVal(char val)

Given 25 horses, you want to find the top 3 horses overall you can race only 5 horses at a time, and got only relative ranking between those 5, not absolute times. How many races you need (minimum) to know who's the top 3 horses