Question

Faced this question in my interview.

What I answerred is that:

1. divide 1b numbers into 10 group

2. Use threadpool to create one thread for each group, 10 totally

3. each thread sum up the result for the group passed and retrun the value

4. use Barrier to sum up all results after 10 threads end, return the final value

My doubt is what’s the expected answer for this question ?
And if it’s running in single-cpu (no multi-thread mode), should single thread be faster than multi-thread ?

Thanks a lot.

Answer 1

A trick question, perhaps – to see if you can think outside the box you’re given?

If they’re consecutive, as noted in the title, and they begin with n, then:

final long BILLION = 1000000000;
long answer = (BILLION * n) + ( BILLION*BILLION + BILLION ) / 2;

This works for n up to somewhere around a billion – after that it will overflow.

Needless to say, I don’t see how multithreading would be of any benefit at all!