寻找中位数（利用快速排序来寻找中位数）

【问题描述】
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.
【输入形式】
Line 1: A single integer N
Lines 2: The input will contain N positive integers that is the milk output of one cow separated by whitespaces .
【输出形式】
Line 1: A single integer that is the median milk output.
【样例输入】
5
2 4 1