FJ is surveying his herd to find the most average cow. He wants
to know how much milk this 'median' cow gives: half of the cows
give as much or more than the median; half give as much or less.
Given an odd number of cows N (1 <= N < 10,000) and their milk output
(1..1,000,000), find the median amount of milk given such that at least
half the cows give the same amount of milk or more and at least half give
the same or less.
输入N个数,输出升序排列后中间那个数.
BZPRO