Could you please give an explanation for this too, Thank you.

Write an O(1) function that takes a single integer, n, as its argument and returns the sum of all integers from 0 through n. (Note: n could be negative. So, for example, sum(-3) should return 0 +-1 +-2 +-3, which is -6.) Credit will only be awarded if your function is O(1). int fastSum(int n)