/// <summary>
/// 交換位置
/// </summary>
/// <param name="v"></param>
/// <param name="index1"></param>
/// <param name="index2"></param>
private void Swrap(int[] v, int index1, int index2)
{
  int temp = v[index1];
  v[index1] = v[index2];
  v[index2] = temp;
}
/// <summary>
/// 將向量V中索引{first,last)劃分成兩個左子表和右子表
/// </summary>
/// <param name="v">向量V</param>
/// <param name="first">開始位置</param>
/// <param name="last">結束位置</param>
private int PivotIndex(int[] v, int first, int last)
{
  if (last == first)
  {
    return last;
  }
  if (last - first == 1)
  {
    return first;
  }
  int mid = (first + last) / 2;
  int midVal = v[mid];
  //交換v[first]和v[mid]
  Swrap(v, first, mid);
  int scanA = first + 1;
  int scanB = last - 1;
  for (; ; )
  {

    while (scanA <= scanB && v[scanA] < midVal)
    {
      scanA++;
    }
    while (scanB > first && midVal <= v[scanB])
    {
      scanB--;
    }
    if (scanA >= scanB)
    {
      break;
    }
    Swrap(v, scanA, scanB);
    scanA++;
    scanB--;
  }
  Swrap(v, first, scanB);
  return scanB;

}

void FindKLargest(int[] v, int first, int last, int k)
{

  //表示分表中值的索引
  int index = 0;
  index = PivotIndex(v, first, last);
  if (index == k)
  {
    //找到了K大
    return;
  }

  if (index > k)
  {
    //只在左子表中查找
    FindKLargest(v, first, index, k);
  }

  else
  {
    //只在右子表中查找
    FindKLargest(v, index, last, k);
  }
}