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Python Program For Recursive Selection Sort For Singly Linked List – Swapping Node Links

  • Last Updated : 18 May, 2022

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.  

sorting image

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. 

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)8:
        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("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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