||2. There is the idea that the TC arena system cannot support more than 50 elements in an array. So much that it became a defacto standard that is enforced even by the local test case window which will not allow you to insert more than 50 elements in an array. Even though the TC system seems to do fine with 150 integers in the last test, and I would dare to say that the strings of 2500 characters are already larger than an array of 500 integers.
Regardless, problems cannot allow more than 50 elements in a single vector. Thus a work around is to send them all in a string.
1. Yet strings are subject to a similar limitation, 50 characters, thus writers use a string of 50 elements to get up to 2500 characters in a string instead of just 50.
On the upside, it still beats having to parse stdin. It also makes test cases easier to wrap in the statistics page.