11/12测试用例，具有帮助功能的Quicksort

Question

This is the test case that does not pass:

5 0 1 8 7

This is my code.

void swap(int* A, int* B)
{
  int t;
  t = *A;
 *A = *B;
 *B=t;
 }

void sorthelper(int * arr,int ind1, int ind2)
{
  int r = ind1+1;
  int w = ind1+1;
  // int i = 0;

  if (ind2 - ind1 <= 1)
    {
      return;
    }

  for(r=ind1+1; r<=ind2;r++)//For r starting at one bigger then pivot and less then the length(ind2), increment r by one each time
     {
       if(arr[r] < arr[ind1])// if read is bigger then pivot, increment w and swap w and r
         { 
           swap(&arr[w],&arr[r]);
           w++;            
         } 
     }
       swap(&arr[ind1], &arr[w-1]);//swap pivot with one less then write spot


       sorthelper(arr, ind1, w-1); 
       sorthelper(arr, w ,ind2); 

}      


void sort(int * arr, int length)
{
  int ind1 = 0;
  int ind2 = 0;
  ind2 = length-1;
  sorthelper(arr, ind1, ind2); 
  return;
}

I am trying to write a quicksort algorithm (Yes, this is hw) and I have EVEYRTHING working except this test case. I've been trying to work this bug out for hours but I have failed. I have tried using GDB to track my values but no luck in determining this error. Can anyone offer any input?

the sort function gets run first, then the search helper is recursive and utilizes the swap function.

Answer 1

Hint: When running your code, you will reach a call like this from the second line of recursive call to sorthelper.

sorthelper([0, 1, 5, 8, 7], 3, 4)

It doesn't do anything although it should sort the 8 and the 7. Think why and fix it ;)

Answer 2

In your sorthelper() function, you are skipping the case when the array has only two elements. Please make the following change:

if (ind2 - ind1 <= 1) 

to

if (ind2 - ind1 < 1)

Without this change, a test case consisting of even two element array would give you an error: (8,7)!

Answer 3

Two problems.

1. You assume that if ind2 - ind1 == 1, the elements are already sorted. This is not true. This is why the [8, 7] partition doesn't end up sorted.
2. When partitioning, you're setting up the lower partition to go from the start of the range to the pivot element (w-1). It should go to the last element BEFORE the pivot (w-2).