For O(n) only way is convert b to hash.

Why isn't the time complexity O(n^2)? For each element in a, you need to scan the whole content of b. (Unless Ruby does something better under the hood).

The time complexity is O(n).
@daniel: Time complexity != execution time.

i am interested as well, but not enough to run some quick benchmarks myself^^

like :)

This is the prefered and idiomatic way

Also, there are extra parentheses around the explode.

Use spoiler flag.

Interesting what is the Time complexity for this solution?

Very elegant, I like the use of the Rust idiom

I think this part of code has a mistake if n == 0 { because the condition says "until the nth year"

consecutive spaces should be removed previously

No. This is solution doesn't waiting for input consecutive space.
Please excuse me, English isn't my native language.