Recursive selection sort for singly linked list | Swapping node links

Given a singly linked list containing n nodes. The problem is to sort the list using recursive selection sort technique. The approach should be such that it involves swapping node links instead of swapping nodes data.

Examples:

Input : 10 -> 12 -> 8 -> 4 -> 6
Output : 4 -> 6 -> 8 -> 10 -> 12

In Selection Sort, we first find minimum element, swap it with the beginning node and recur for remaining list. Below is recursive implementation of these steps for linked list.

recurSelectionSort(head)
     if head->next == NULL
         return head
     Initialize min = head
     Initialize beforeMin = NULL
     Initialize ptr = head
    
     while ptr->next != NULL 
         if min->data > ptr->next->data
         min = ptr->next
         beforeMin = ptr
     ptr = ptr->next    
    
     if min != head
         swapNodes(&head, head, min, beforeMin)
    
     head->next = recurSelectionSort(head->next)
     return head

swapNodes(head_ref, currX, currY, prevY)
     head_ref = currY
     prevY->next = currX

     Initialize temp = currY->next
     currY->next = currX->next
     currX->next  = temp    

void swapNodes(struct Node** head_ref, struct Node* currX,

               struct Node* currY, struct Node* prevY)

{

    // make 'currY' as new head

    *head_ref = currY;

 

    // adjust links

    prevY->next = currX;

 

    // Swap next pointers

    struct Node* temp = currY->next;

    currY->next = currX->next;

    currX->next = temp;

}

 

// function to sort the linked list using

// recursive selection sort technique

struct Node* recurSelectionSort(struct Node* head)

{

    // if there is only a single node

    if (head->next == NULL)

        return head;

 

    // 'min' - pointer to store the node having

    // minimum data value

    struct Node* min = head;

 

    // 'beforeMin' - pointer to store node previous

    // to 'min' node

    struct Node* beforeMin = NULL;

    struct Node* ptr;

 

    // traverse the list till the last node

    for (ptr = head; ptr->next != NULL; ptr = ptr->next) {

 

        // if true, then update 'min' and 'beforeMin'

        if (ptr->next->data < min->data) {

            min = ptr->next;

            beforeMin = ptr;

        }

    }

 

    // if 'min' and 'head' are not same,

    // swap the head node with the 'min' node

    if (min != head)

        swapNodes(&head, head, min, beforeMin);

 

    // recursively sort the remaining list

    head->next = recurSelectionSort(head->next);

 

    return head;

}

 

// function to sort the given linked list

void sort(struct Node** head_ref)

{

    // if list is empty

    if ((*head_ref) == NULL)

        return;

 

    // sort the list using recursive selection

    // sort technique

    *head_ref = recurSelectionSort(*head_ref);

}