原题如下：
Description

There are two sequences A and B with N (1<=N<=10000) elements each. All of the elements are positive integers. Given C=A*B, where '*' representing Cartesian product, c = a*b, where c belonging to C, a belonging to A and b belonging to B. Your job is to find the K'th largest element in C, where K begins with 1.

Input

Input file contains multiple test cases. The first line is the number of test cases. There are three lines in each test case. The first line of each case contains two integers N and K, then the second line represents elements in A and the following line represents elements in B. All integers are positive and no more than 10000. K may be more than 10000.

Output

For each case output the K'th largest number.

Sample Input

2
2 1
3 4
5 6
2 3
2 1
4 8
Sample Output

24
8

using System;

namespace CSharpTest
{
    class Program
    {
        public static int[] A = new int[1000000], B = new int[1000000];
        public static Random Rnd = new Random(2010);//统一一个随机种子好验证

        public static void FillRandomValues(int[] arr)//随机填充
        {
            for (var i = 0; i < arr.Length; i++)
                arr[i] = Rnd.Next();
        }

        public static void Main()
        {
            FillRandomValues(A);
            FillRandomValues(B);
            //排序
            Array.Sort<int>(A);
            Array.Sort<int>(B);

            //根据输入的K输出结果
            while (true)
            {
                long k;
                if (long.TryParse(Console.ReadLine(), out k))
                {
                    if (k == 0) break;
                    Console.WriteLine(KthElement(k));
                }
            }
        }

        //求第K个元素
        static long KthElement(long k)
        {
            //该数肯定位于upper和lower之间
            long upper = (long)A[A.Length - 1] * B[B.Length - 1], lower = A[0] * B[0];
            return Solve(upper, lower, k);
        }

        //二分查找该值
        static long Solve(long upper, long lower, long count)
        {
            if (upper == lower) return upper;

            long mid = (upper + lower) >> 1;

            if (Counter(mid) >= count)
                return Solve(upper, mid + 1, count);

            if (Counter(mid - 1) < count)
                return Solve(mid - 1, lower, count);

            return mid;
        }

        //统计比指定value大的数有多少个
        static long Counter(long value)
        {
            int i = 0, j = B.Length - 1;
            long count = 0;

            while (i < A.Length && j >= 0)
            {
                if ((long)A[i] * (long)B[j] > value)
                {
                    count += A.Length - i;
                    j--;
                }
                else
                    i++;
            }

            return count;
        }
    }
}

------解决方案--------------------
用堆排序求第K大数据比较好

------解决方案--------------------