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  Quick Sort On Linked List
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I’ve been making a number of standard classes for my own future developments and one of them was a linked list class. I wanted a method to sort the list but not using the slow bubble sort. I wanted the speed of quick sort, but there wasn’t a qsort run time function for linked lists. So I hunted down the quick sort algorithm and applied it to my linked list class.

Anyway, I wanted to give something useful to COTD, so I quickly wrote the code below to show you how its done. I haven’t compared the function to a quick sort function using an array, so I don’t know how fast it is.

Download Associated File: qsort.cpp (3,656 bytes)

// Quick Sort on a Linked List
// By Stephen Hassall
// Web:
// EMail:

#include <stdio.h>

// Define a linked list structure typedef struct _tagIntegerList { // The integer object int nInteger;

// Pointers struct _tagIntegerList *pPrev; struct _tagIntegerList *pNext; } IntegerList;

// Pointers to First and Last items IntegerList *g_pFirstItem = NULL; IntegerList *g_pLastItem = NULL;

// Add List Item void AddListItem(int nInteger) { // Create a new list item IntegerList *pItem = new IntegerList; // Set Integer value pItem->nInteger = nInteger;

// Add to list // If this is the first item added if (g_pFirstItem == NULL) { g_pFirstItem = g_pLastItem = pItem; pItem->pNext = pItem->pPrev = NULL; } else { // Add item to the end of the list g_pLastItem->pNext = pItem; pItem->pPrev = g_pLastItem; g_pLastItem = pItem; pItem->pNext = NULL; } }

// Remove all items void RemoveAllItems() { IntegerList *pDelItem, *pItem = g_pFirstItem;

// Loop while there are items while (pItem != NULL) { // Set the delete item pDelItem = pItem;

// Move to the next item pItem = pItem->pNext;

// Delete the item delete pDelItem; }

// Set global pointers to NULL g_pFirstItem = g_pLastItem = NULL; }

// Print List void PrintList() { IntegerList *pItem = g_pFirstItem;

// Loop while there are items while (pItem != NULL) { // Print this item printf("%d\n", pItem->nInteger);

// Move to the next item pItem = pItem->pNext; } }

// Quick Sort List void QuickSortList(IntegerList *pLeft, IntegerList *pRight) { IntegerList *pStart; IntegerList *pCurrent; int nCopyInteger;

// If the left and right pointers are the same, then return if (pLeft == pRight) return;

// Set the Start and the Current item pointers pStart = pLeft; pCurrent = pStart->pNext;

// Loop forever (well until we get to the right) while (1) { // If the start item is less then the right if (pStart->nInteger < pCurrent->nInteger) { // Swap the items nCopyInteger = pCurrent->nInteger; pCurrent->nInteger = pStart->nInteger; pStart->nInteger = nCopyInteger; } // Check if we have reached the right end if (pCurrent == pRight) break;

// Move to the next item in the list pCurrent = pCurrent->pNext; }

// Swap the First and Current items nCopyInteger = pLeft->nInteger; pLeft->nInteger = pCurrent->nInteger; pCurrent->nInteger = nCopyInteger;

// Save this Current item IntegerList *pOldCurrent = pCurrent;

// Check if we need to sort the left hand size of the Current point pCurrent = pCurrent->pPrev; if (pCurrent != NULL) { if ((pLeft->pPrev != pCurrent) && (pCurrent->pNext != pLeft)) QuickSortList(pLeft, pCurrent); }

// Check if we need to sort the right hand size of the Current point pCurrent = pOldCurrent; pCurrent = pCurrent->pNext; if (pCurrent != NULL) { if ((pCurrent->pPrev != pRight) && (pRight->pNext != pCurrent)) QuickSortList(pCurrent, pRight); } }

// Main void main () { // Add some items AddListItem(100); AddListItem(12); AddListItem(56); AddListItem(67); AddListItem(4); AddListItem(91); AddListItem(34); AddListItem(59); AddListItem(42); AddListItem(20); AddListItem(83); AddListItem(74); AddListItem(33); AddListItem(79); AddListItem(49); AddListItem(51);

// Sort the list QuickSortList(g_pFirstItem, g_pLastItem);

// Print list PrintList();

// Remove items from list RemoveAllItems(); }

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