/// <summary>
/// 找到一组数中第k小元素 最坏时间复杂度O^2 期望时间复杂度O(n)
/// </summary>
/// <typeparam name="T"></typeparam>
/// <param name="a"></param>
/// <param name="k"></param>
/// <returns></returns>
public static T Select <T>(T[] a, int k)
{
RandomAlgorithm.Shuffle <T>(a);
int lo = 0, hi = a.Length - 1;
while (hi > lo)
{
int j = QuickSort <T> .Partition(a, lo, hi);
if (j == k)
{
return(a[k]);
}
else if (j > k)
{
hi = j - 1;
}
else if (j < k)
{
lo = j + 1;
}
}
return(a[k]);
}
public static void Sort(T[] a)
{
RandomAlgorithm.Shuffle <T>(a);
Sort(a, 0, a.Length - 1);
}
