Find the longest sequence length whose sum is divisible by 3

Question

I have an exercise that needs to be done with O(n) time complexity, however, I can only solve it with an O(n^2) solution.

You have an array and you need to count the

Answer 1

As a non-CS person, this is interesting. First approach of mine was simply to calc the running sum mod 3. You'll get a sequence of {0,1,2}. Now look for the first and the last 0, the first and the last 1 and the first and the last 2, and compare their respective distances...