快排演算法在分治的時候有兩種實現，一種實現是從兩邊到中間(partition)，另一種實現是從一邊到另一邊(partition2)。我用一個100000陣列測試發現前一種實現執行速度快一些。

這兩種的C++實現如下： （注：我用的程式碼風格是gnu的程式碼風格）

bool sort::qsort(int *ini, int start, int end)
{ // sort the array ini by ascending order
  int tmp=0;
  int mid=0;

  if (end < start){
    return false;
  }else if(end == start){
    if(ini[start] > ini[end])
      {
	tmp = ini[start];
	ini[start] = ini[end];
	ini[end] = tmp;
      }
    return true;
  }else{			   // when start is less end;
    mid = this->partition2(ini, start, end); 
    // the partition fun. can either be partiton or partiton2.

    if(mid == start && qsort(ini,mid+1,end))
      return true;

    if( mid == end && qsort(ini,start,mid-1))
      return true;

    if(qsort(ini,start,mid-1) && qsort(ini,mid+1,end))
      return true;
    return true;
  }
}

int sort::partition(int *ini, int start, int end)
{   // the main body for partition which from edge to middle
  int key=0;
  int i=start;
  int j=end;
  int tmp = 0;
  key = ini[i];

  while(i!=j)
    {
      while(key <= ini[j] && i < j)
	j--;

      // exchange value, from right to left
      tmp = ini[i];
      ini[i] = ini[j];
      ini[j] = tmp;

      while(key >= ini[i] && i < j)
	i++;

      // exchange value, from left to right
      tmp = ini[i];
      ini[i] = ini[j];
      ini[j] = tmp;
    }

  return i;
}

int sort::partition2(int *ini, int start, int end)
{   // the main body for partition which from end to begin
  int key = ini[end];
  int i = start - 1;		// suppose the mid pos is i+1
  int j=start;
  int tmp = 0;

  for(;j<end;j++)
    {
      if(ini[j] < key )
	{
	  i++;
	  tmp = ini[i];
	  ini[i] = ini[j];
	  ini[j] = tmp;
	}
    }
  ini[end] = ini[i+1];
  ini[i+1] = key;
  return i+1;
}