算法-快速排序的实现问题

Question

I am learning algorithms as a beginner.When learning quicksort,I found that there're several ways to implement the quick sort.So I choose this the way in this tutorial to implement quicksort.

It chooses the element in end of the array as pivot,and choose the first element as a wall separating the values which are less and greater than the pivot.when the sort is done, insert the pivot in the position of wall and make partition.

Here's my implementation in c:

void simpleqs(int* data,int start,int end)
{
 int wall = start;
 int pivot = data[end];

 //Base case
 if (start >= end)
 {
   return;
 }

 //Sort the array
 for(int e = start;e < end-1; e++)
 {
   if(data[e] < pivot)
   {
    swap(&data[e],&data[wall]);
    wall++;
   }
 }

 //Partition

 if(data[wall] >= pivot)
 {
   swap(&data[wall],&data[end]);
 }

 simpleqs(data,start,wall-1);
 simpleqs(data,wall+1,end);
 }

In main:

int main(void)
{
  int dataSet[] = {2,4,1,5,6,9,8,3,7,10,20,13,11,17,15};

  int size = sizeof(dataSet)/sizeof(int);

  simpleqs(dataSet,0,size-1);

  for(int e = 0;e < size ; e++)
  {
    printf("%d,", dataSet[e]);
  }
  printf("\n");

}

There will always be one value in the wrong position,I can't figure out why.

Like this:

1,2,3,4,5,6,8,9,7,10,11,13,15,17,20,

Please help me to revise my logic,thanks!

Answer 1

You should go to the end, not end - 1.

// Sort the array
for (int e = start; e < end; e++)
{
    if (data[e] < pivot)
    {
        swap(&data[e], &data[wall]);
        wall++;
    }
}

Answer 2

您已经实现了Lomuto分区 ，但是错过了对(end-1) -th元素的处理：

for(int e = start;e < end; e++)