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 the recursive selection sort technique. The approach should be such that it involves swapping node links instead of swapping node data.  

Examples:

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

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

Implementation:

C++

// 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;
}

Java

// 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

Python

# Python implementation of recursive selection sort
# for singly linked list | Swapping node links

# Linked List node
class Node:
    def __init__(self, data):
        self.data = data
        self.next = None

# function to swap nodes 'currX' and 'currY' in a
# linked list without swapping data
def swapNodes(head_ref, currX, currY, prevY) :

    # make 'currY' as new head
    head_ref = currY

    # adjust links
    prevY.next = currX

    # Swap next pointers
    temp = currY.next
    currY.next = currX.next
    currX.next = temp
    return head_ref

# function to sort the linked list using
# recursive selection sort technique
def recurSelectionSort( head) :

    # if there is only a single node
    if (head.next == None) :
        return head

    # 'min' - pointer to store the node having
    # minimum data value
    min = head

    # 'beforeMin' - pointer to store node previous
    # to 'min' node
    beforeMin = None
    ptr = head

    # traverse the list till the last node
    while ( ptr.next != None ) :

        # if true, then update 'min' and 'beforeMin'
        if (ptr.next.data < min.data) :

            min = ptr.next
            beforeMin = ptr

        ptr = ptr.next

    # 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
def sort( head_ref) :

    # if list is empty
    if ((head_ref) == None) :
        return None

    # 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
def push( head_ref, new_data) :

    # allocate node
    new_node = Node(0)

    # 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
def printList( head) :

    while (head != None) :

        print( head.data ,end = " ")
        head = head.next

# Driver code
head = None

# 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)

print( "Linked list before sorting:")
printList(head)

# sort the linked list
head = sort(head)

print( "\nLinked list after sorting:")
printList(head)

# This code is contributed by Arnab Kundu

C#

// C# implementation of recursive selection sort
// for singly linked list | Swapping node links
using System;
public class GFG
{

// A Linked list node
public class Node
{
    public int data;
    public 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)
    {
        Console.Write(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);

    Console.WriteLine("Linked list before sorting:");
    printList(head);

    // sort the linked list
    head = sort(head);

    Console.Write("\nLinked list after sorting:");
    printList(head);
}
}


// This code is contributed by Princi Singh

Javascript

<script>
// javascript implementation of recursive selection sort
// for singly linked list | Swapping node links     // A Linked list node
     class Node {
        constructor(val) {
            this.data = val;
            this.next = null;
        }
    }

    // function to swap nodes 'currX' and 'currY' in a
    // linked list without swapping data
    function swapNodes(head_ref,  currX,  currY,  prevY) {
        // make 'currY' as new head
        head_ref = currY;

        // adjust links
        prevY.next = currX;

        // Swap next pointers
        var temp = currY.next;
        currY.next = currX.next;
        currX.next = temp;
        return head_ref;
    }

    // function to sort the linked list using
    // recursive selection sort technique
    function recurSelectionSort(head) {
        // if there is only a single node
        if (head.next == null)
            return head;

        // 'min' - pointer to store the node having
        // minimum data value
        var min = head;

        // 'beforeMin' - pointer to store node previous
        // to 'min' node
        var beforeMin = null;
        var 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
    function sort(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
    function push(head_ref , new_data) {
        // allocate node
        var 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
    function printList(head) {
        while (head != null) {
            document.write(head.data + " ");
            head = head.next;
        }
    }

    // Driver code

        var 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);

        document.write("Linked list before sorting:<br/>");
        printList(head);

        // sort the linked list
        head = sort(head);

        document.write("<br/>Linked list after sorting:<br/>");
        printList(head);

// This code is contributed by todaysgaurav
</script>

Output

Linked list before sorting:n10 12 8 4 6 
Linked list after sorting:n4 6 8 10 12 

Time Complexity: O(n²)
Auxiliary Space: O(n)