Alice is trying to keep track of her sleeping pattern over a period of
n days.  Let A[i] be the number of hours Alice slept in the i-th day.
While the number of hours of sleep Alice gets in each day varies quite
a bit, she hopes that the average over any k consecutive days is not
too small.

Give an O(n) algorithm that finds the "worst" interval of k days in
Alice's schedule.  That is, the set of k consecutive days for which
Alice has slept the least over all such periods.

Note that the bound O(n) is independent of k!