QuickSort on Singly Linked List

// C++ program for Quick Sort on Singly Linled List
#include <iostream>
#include <cstdio>
using namespace std;
 
/* a node of the singly linked list */
struct node
{
    int data;
    struct node *next;
};
 
/* A utility function to insert a node at the beginning of linked list */
void push(struct node** head_ref, int new_data)
{
    /* allocate node */
    struct node* new_node = new node;
 
    /* put in the data  */
    new_node->data  = new_data;
 
    /* link the old list off the new node */
    new_node->next = (*head_ref);
 
    /* move the head to point to the new node */
    (*head_ref)    = new_node;
}
 
/* A utility function to print linked list */
void printList(struct node *node)
{
    while (node != NULL)
    {
        printf("%d  ", node->data);
        node = node->next;
    }
    printf("\n");
}
 
// Returns the last node of the list
struct node *getTail(struct node *cur)
{
    while (cur != NULL && cur->next != NULL)
        cur = cur->next;
    return cur;
}
 
// Partitions the list taking the last element as the pivot
struct node *partition(struct node *head, struct node *end,
                       struct node **newHead, struct node **newEnd)
{
    struct node *pivot = end;
    struct node *prev = NULL, *cur = head, *tail = pivot;
 
    // During partition, both the head and end of the list might change
    // which is updated in the newHead and newEnd variables
    while (cur != pivot)
    {
        if (cur->data < pivot->data)
        {
            // First node that has a value less than the pivot - becomes
            // the new head
            if ((*newHead) == NULL)
                (*newHead) = cur;
 
            prev = cur;  
            cur = cur->next;
        }
        else // If cur node is greater than pivot
        {
            // Move cur node to next of tail, and change tail
            if (prev)
                prev->next = cur->next;
            struct node *tmp = cur->next;
            cur->next = NULL;
            tail->next = cur;
            tail = cur;
            cur = tmp;
        }
    }
 
    // If the pivot data is the smallest element in the current list,
    // pivot becomes the head
    if ((*newHead) == NULL)
        (*newHead) = pivot;
 
    // Update newEnd to the current last node
    (*newEnd) = tail;
 
    // Return the pivot node
    return pivot;
}
 
 
//here the sorting happens exclusive of the end node
struct node *quickSortRecur(struct node *head, struct node *end)
{
    // base condition
    if (!head || head == end)
        return head;
 
    node *newHead = NULL, *newEnd = NULL;
 
    // Partition the list, newHead and newEnd will be updated
    // by the partition function
    struct node *pivot = partition(head, end, &newHead, &newEnd);
 
    // If pivot is the smallest element - no need to recur for
    // the left part.
    if (newHead != pivot)
    {
        // Set the node before the pivot node as NULL
        struct node *tmp = newHead;
        while (tmp->next != pivot)
            tmp = tmp->next;
        tmp->next = NULL;
 
        // Recur for the list before pivot
        newHead = quickSortRecur(newHead, tmp);
 
        // Change next of last node of the left half to pivot
        tmp = getTail(newHead);
        tmp->next =  pivot;
    }
 
    // Recur for the list after the pivot element
    pivot->next = quickSortRecur(pivot->next, newEnd);
 
    return newHead;
}
 
// The main function for quick sort. This is a wrapper over recursive
// function quickSortRecur()
void quickSort(struct node **headRef)
{
    (*headRef) = quickSortRecur(*headRef, getTail(*headRef));
    return;
}
 
// Driver program to test above functions
int main()
{
    struct node *a = NULL;
    push(&a, 5);
    push(&a, 20);
    push(&a, 4);
    push(&a, 3);
    push(&a, 30);
 
    cout << "Linked List before sorting \n";
    printList(a);
 
    quickSort(&a);
 
    cout << "Linked List after sorting \n";
    printList(a);
 
    return 0;
}