Java Program For 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. 

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( 
    "Linked list after sorting:"); 
    printList(head); 
} 
} 
// This code is contributed by Arnab Kundu

Output:

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

Time Complexity: O(n²)

Auxiliary Space: O(n)

Please refer complete article on Recursive selection sort for singly linked list | Swapping node links for more details!

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