You have a list of servers numbered with nonnegative integers, like this: [0, 1, 5, 4, 2]
You have n + 1 servers numbered 0, 1, 2, 3...n
If they fail, we have to bring up a new server
Each new server has to be numbered with the lowest possilbe number
Write a function that returns the smallest nonnegative missing integer from input array, and if none are missing, return the next number in the sequence.
So it would return 3 for the above input.