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.
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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    

The swapNodes(head_ref, currX, currY, prevY) is based on the approach discussed here but it is modified accordingly for the implementation of this post.

C++

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// C++ implementation of recursive selection sort
// for singly linked list | Swapping node links
#include <bits/stdc++.h>
using namespace std;
  
// A Linked list node
struct Node {
    int data;
    struct Node* next;
};
  
// function to swap nodes 'currX' and 'currY' in a
// linked list without swapping data
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);
}
  
// function to insert a node at the
// beginning of the linked list
void push(struct Node** head_ref, int new_data)
{
    // allocate node
    struct Node* new_node = 
         (struct Node*)malloc(sizeof(struct Node));
  
    // put in the data
    new_node->data = new_data;
  
    // link the old list to the new node
    new_node->next = (*head_ref);
  
    // move the head to point to the new node
    (*head_ref) = new_node;
}
  
// function to print the linked list
void printList(struct Node* head)
{
    while (head != NULL) {
        cout << head->data << " ";
        head = head->next;
    }
}
  
// Driver program to test above
int main()
{
    struct Node* head = NULL;
  
    // create linked list 10->12->8->4->6
    push(&head, 6);
    push(&head, 4);
    push(&head, 8);
    push(&head, 12);
    push(&head, 10);
  
    cout << "Linked list before sorting:n";
    printList(head);
  
    // sort the linked list
    sort(&head);
  
    cout << "\nLinked list after sorting:n";
    printList(head);
  
    return 0;
}

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Java

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// Java implementation of recursive selection sort 
// for singly linked list | Swapping node links 
class GFG
{
      
// A Linked list node 
static class Node
    int data; 
    Node next; 
}; 
  
// function to swap nodes 'currX' and 'currY' in a 
// linked list without swapping data 
static Node swapNodes( Node head_ref, Node currX, 
                        Node currY, Node prevY) 
    // make 'currY' as new head 
    head_ref = currY; 
  
    // adjust links 
    prevY.next = currX; 
  
    // Swap next pointers 
    Node temp = currY.next; 
    currY.next = currX.next; 
    currX.next = temp; 
    return head_ref;
  
// function to sort the linked list using 
// recursive selection sort technique 
static Node recurSelectionSort( 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 
    Node min = head; 
  
    // 'beforeMin' - pointer to store node previous 
    // to 'min' node 
    Node beforeMin = null
    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) 
        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 
static Node sort( Node head_ref) 
    // if list is empty 
    if ((head_ref) == null
        return null
  
    // sort the list using recursive selection 
    // sort technique 
    head_ref = recurSelectionSort(head_ref); 
    return head_ref;
  
// function to insert a node at the 
// beginning of the linked list 
static Node push( Node head_ref, int new_data) 
    // allocate node 
    Node new_node = new Node(); 
  
    // put in the data 
    new_node.data = new_data; 
  
    // link the old list to the new node 
    new_node.next = (head_ref); 
  
    // move the head to point to the new node 
    (head_ref) = new_node; 
    return head_ref;
  
// function to print the linked list 
static void printList( Node head) 
    while (head != null
    
        System.out.print( head.data + " "); 
        head = head.next; 
    
  
// Driver code 
public static void main(String args[])
    Node head = null
  
    // create linked list 10.12.8.4.6 
    head = push(head, 6); 
    head = push(head, 4); 
    head = push(head, 8); 
    head = push(head, 12); 
    head = push(head, 10); 
  
    System.out.println( "Linked list before sorting:"); 
    printList(head); 
  
    // sort the linked list 
    head = sort(head); 
  
    System.out.print( "\nLinked list after sorting:"); 
    printList(head); 
  
// This code is contributed by Arnab Kundu

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Output:

Linked list before sorting:
10 12 8 4 6
Linked list after sorting:
4 6 8 10 12

Time Complexity: O(n2)

This article is contributed by Ayush Jauhari. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

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