I read through all previous post regarding quicksort on a linked list. The trouble I am having is using a random element, it seems all the other post are using the first or middle element where I want to use a random element for the pivot.
in anycase here is my shot at it.
#include <stdlib.h> //rand, malloc
#include <stdio.h> //printf
struct listnode {
struct listnode *next;
long value;
};
//Finds length of list, which is usefull in selecting a random pivot
int ListLength (struct listnode *list)
{
struct listnode *temp = list;
int i=0;
while(temp!=NULL)
{
i++;
temp=temp->next;
}
return i;
}
// Prints list to help while working on homework
void printList(struct listnode *list)
{
struct listnode *node;
node=list;
printf("\nList Values\n");
while(node!=NULL)
{
printf("%2lo ", node->value);
node=node->next;
}
}
// Creates a list of desired length
struct listnode *createList(int lengthOfList)
{
long i;
struct listnode *node, *space;
space = (struct listnode *) malloc( lengthOfList*sizeof(struct listnode));
for( i=0; i< lengthOfList; i++ )
{ (space + i)->value = 2*((17*i+1)%lengthOfList);
(space + i)->next = space + (i+1);
}
(space+(lengthOfList-1))->next = NULL;
node = space;
return node;
}
// Prof Brass's test
void Brass_test(struct listnode *list)
{
int i;
printf("\nChecking sorted list\n");
for( i=0; i < 100; i++)
{
if( list == NULL )
{
printf("List ended early\n"); exit(0);
}
if( list->value != 2*i )
{
printf("Node contains wrong value\n"); exit(0);
}
list = list->next;
}
printf("Sort successful\n");
}
// Selects a random pivot point
struct listnode *SelectPivot(struct listnode *list)
{
int k, n, i = 0;
n = ListLength(list);
struct listnode *pivot=list;
k=rand()%n;
for (; i < k; ++i)
{
pivot=pivot->next;
}
return pivot;
}
// Sorts a list using quicksort algo with random pivot point
struct listnode *Quicksort(struct listnode *list)
{
// Return NULL list
if (ListLength(list) <= 1) return list;
struct listnode *less=NULL, *more=NULL, *next, *endl, *temp=list;
// Select a random pivot point
struct listnode *pivot = SelectPivot(list);
printf("Pivot Value = %lo\n", pivot->value);
// Divide & Conquer
while(temp != NULL)
{
next = temp->next;
if(temp->value < pivot->value)
{
temp->next = less;
less = temp;
}
else
{
temp->next = more;
more = temp;
}
temp = next;
}
// printf("\nLess list =\n");
// printf("List Count %d\n", ListLength(less));
// printList(less);
// printf("\nMore list =\n");
// printf("List Count %d\n", ListLength(more));
// printList(more);
// printf("\nlist list =\n");
// printf("List Count %d\n", ListLength(list));
// printList(list);
// Recursive Calls
// less = Quicksort(less);
// more = Quicksort(more);
// Merge
if(ListLength(less)!=0)
{
while(endl != NULL)
{
endl = less->next;
less->next = more;
more = less;
less = endl;
}
return more;
}
else
{
return more;
}
}
int main(void)
{
struct listnode *node;
node = createList(25);
printf("Unsorted List\n");
printList(node);
printf("\nSorted List\n");
node = Quicksort(node);
printf("\nList Count node %d\n", ListLength(node));
printList(node);
// Brass_test(node);
exit(0);
}
Thanks in advance.