Iterative selection sort for linked list

Given a linked list, the task is to sort the linked list in ascending order by using selection sort.

Examples:

Input : 1->4->2->2->3
Output : 1->2->2->3->4

Input : 5->4->3->2
Output : 2->3->4->5


Selection Sort Algorithm: Iterate the given list N times where N is the number of elements in the list. In every iteration of selection sort, the minimum element (considering ascending order) from the unsorted subarray is picked and moved to the sorted subarray.

Example:

list = 64 25 12 22 11

// Find the minimum element in list(0...4)
// and place it at beginning
11 25 12 22 64

// Find the minimum element in list(1...4)
// and place it at beginning of list(1...4)
11 12 25 22 64

// Find the minimum element in list(2...4)
// and place it at beginning of list(2...4)
11 12 22 25 64

// Find the minimum element in list(3...4)
// and place it at beginning of list(3...4)
11 12 22 25 64 

The swapping required can be done in two ways:

  1. By swapping the data parts of the nodes.
  2. By swapping the complete nodes.

Second implementation is generally used when elements of the list are some kind of records because in such a case data swapping becomes tedious and expensive due to the presence of a large number of data elements.

Implementation Method 1: Below is the implementation of selection sort function for sorting linked lists by swapping only data parts of a node.

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void selectionSort(node* head)
{
    node* temp = head;
  
    // Traverse the List
    while (temp) {
        node* min = temp;
        node* r = temp->next;
  
        // Traverse the unsorted sublist
        while (r) {
            if (min->data > r->data)
                min = r;
  
            r = r->next;
        }
  
        // Swap Data
        int x = temp->data;
        temp->data = min->data;
        min->data = x;
        temp = temp->next;
    }
}

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Method 2: Data swapping is no doubt easier to implement and understand, but in some cases( as one mentioned above ), it isn’t desirable. While doing swapping of the next parts of two nodes, four cases are needed to be taken into consideration :

  1. Nodes are adjacent and the first node is the starting node.
  2. Nodes are adjacent and the first node isn’t the starting node.
  3. Nodes aren’t adjacent and the first node is the starting node.
  4. Nodes aren’t adjacent and the first node isn’t the starting node.

Below is the implementation of the above approach:

C++

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// C++ implementation of the approach
#include <bits/stdc++.h>
using namespace std;
  
// Linked List Node
struct Node {
    int data;
    Node* next;
};
  
// Utility function to create a
// new Linked List Node
Node* newNode(int val)
{
    Node* temp = new Node;
  
    temp->data = val;
  
    temp->next = NULL;
  
    return temp;
}
  
// Function to sort a linked list using selection
// sort algorithm by swapping the next pointers
Node* selectionSort(Node* head)
{
    Node *a, *b, *c, *d, *r;
  
    a = b = head;
  
    // While b is not the last node
    while (b->next) {
  
        c = d = b->next;
  
        // While d is pointing to a valid node
        while (d) {
  
            if (b->data > d->data) {
  
                // If d appears immediately after b
                if (b->next == d) {
  
                    // Case 1: b is the head of the linked list
                    if (b == head) {
  
                        // Move d before b
                        b->next = d->next;
                        d->next = b;
  
                        // Swap b and d pointers
                        r = b;
                        b = d;
                        d = r;
  
                        c = d;
  
                        // Update the head
                        head = b;
  
                        // Skip to the next element
                        // as it is already in order
                        d = d->next;
                    }
  
                    // Case 2: b is not the head of the linked list
                    else {
  
                        // Move d before b
                        b->next = d->next;
                        d->next = b;
                        a->next = d;
  
                        // Swap b and d pointers
                        r = b;
                        b = d;
                        d = r;
  
                        c = d;
  
                        // Skip to the next element
                        // as it is already in order
                        d = d->next;
                    }
                }
  
                // If b and d have some non-zero
                // number of nodes in between them
                else {
  
                    // Case 3: b is the head of the linked list
                    if (b == head) {
  
                        // Swap b->next and d->next
                        r = b->next;
                        b->next = d->next;
                        d->next = r;
                        c->next = b;
  
                        // Swap b and d pointers
                        r = b;
                        b = d;
                        d = r;
  
                        c = d;
  
                        // Skip to the next element
                        // as it is already in order
                        d = d->next;
  
                        // Update the head
                        head = b;
                    }
  
                    // Case 4: b is not the head of the linked list
                    else {
  
                        // Swap b->next and d->next
                        r = b->next;
                        b->next = d->next;
                        d->next = r;
                        c->next = b;
                        a->next = d;
  
                        // Swap b and d pointers
                        r = b;
                        b = d;
                        d = r;
  
                        c = d;
  
                        // Skip to the next element
                        // as it is already in order
                        d = d->next;
                    }
                }
            }
            else {
  
                // Update c and skip to the next element
                // as it is already in order
                c = d;
                d = d->next;
            }
        }
  
        a = b;
        b = b->next;
    }
  
    return head;
}
  
// Function to print the list
void printList(Node* head)
{
    while (head) {
        cout << head->data << " ";
        head = head->next;
    }
}
  
// Driver Code
int main()
{
    Node* head = newNode(5);
    head->next = newNode(4);
    head->next->next = newNode(3);
  
    head = selectionSort(head);
  
    printList(head);
  
    return 0;
}

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Java

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// Java implementation of the approach 
class GFG
{
      
// Linked List Node 
static class Node
    int data; 
    Node next; 
}; 
  
// Utility function to create a 
// new Linked List Node 
static Node newNode(int val) 
    Node temp = new Node(); 
  
    temp.data = val; 
  
    temp.next = null
  
    return temp; 
  
// Function to sort a linked list using selection 
// sort algorithm by swapping the next pointers 
static Node selectionSort(Node head) 
    Node a, b, c, d, r; 
  
    a = b = head; 
  
    // While b is not the last node 
    while (b.next != null
    
  
        c = d = b.next; 
  
        // While d is pointing to a valid node 
        while (d != null)  
        
  
            if (b.data > d.data)
            
  
                // If d appears immediately after b 
                if (b.next == d) 
                
  
                    // Case 1: b is the head of the linked list 
                    if (b == head) 
                    
  
                        // Move d before b 
                        b.next = d.next; 
                        d.next = b; 
  
                        // Swap b and d pointers 
                        r = b; 
                        b = d; 
                        d = r; 
  
                        c = d; 
  
                        // Update the head 
                        head = b; 
  
                        // Skip to the next element 
                        // as it is already in order 
                        d = d.next; 
                    
  
                    // Case 2: b is not the head of the linked list 
                    else 
                    
  
                        // Move d before b 
                        b.next = d.next; 
                        d.next = b; 
                        a.next = d; 
  
                        // Swap b and d pointers 
                        r = b; 
                        b = d; 
                        d = r; 
  
                        c = d; 
  
                        // Skip to the next element 
                        // as it is already in order 
                        d = d.next; 
                    
                
  
                // If b and d have some non-zero 
                // number of nodes in between them 
                else 
                
  
                    // Case 3: b is the head of the linked list 
                    if (b == head) 
                    
  
                        // Swap b.next and d.next 
                        r = b.next; 
                        b.next = d.next; 
                        d.next = r; 
                        c.next = b; 
  
                        // Swap b and d pointers 
                        r = b; 
                        b = d; 
                        d = r; 
  
                        c = d; 
  
                        // Skip to the next element 
                        // as it is already in order 
                        d = d.next; 
  
                        // Update the head 
                        head = b; 
                    
  
                    // Case 4: b is not the head of the linked list 
                    else 
                    
  
                        // Swap b.next and d.next 
                        r = b.next; 
                        b.next = d.next; 
                        d.next = r; 
                        c.next = b; 
                        a.next = d; 
  
                        // Swap b and d pointers 
                        r = b; 
                        b = d; 
                        d = r; 
  
                        c = d; 
  
                        // Skip to the next element 
                        // as it is already in order 
                        d = d.next; 
                    
                
            
            else 
            
  
                // Update c and skip to the next element 
                // as it is already in order 
                c = d; 
                d = d.next; 
            
        
  
        a = b; 
        b = b.next; 
    
  
    return head; 
  
// Function to print the 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 = newNode(5); 
    head.next = newNode(4); 
    head.next.next = newNode(3); 
  
    head = selectionSort(head); 
  
    printList(head); 
}
  
// This code is contributed by Arnab Kundu

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Python3

# Python3 implementation of the approach

# Linked List Node
class Node:

def __init__(self, val):
self.data = val
self.next = None

# Function to sort a linked list
# using selection sort algorithm
# by swapping the next pointers
def selectionSort(head):

a = b = head

# While b is not the last node
while b.next:

c = d = b.next

# While d is pointing to a valid node
while d:

if b.data > d.data:

# If d appears immediately after b
if b.next == d:

# Case 1: b is the head
# of the linked list
if b == head:

# Move d before b
b.next = d.next
d.next = b

# Swap b and d pointers
b, d = d, b
c = d

# Update the head
head = b

# Skip to the next element
# as it is already in order
d = d.next

# Case 2: b is not the head
# of the linked list
else:

# Move d before b
b.next = d.next
d.next = b
a.next = d

# Swap b and d pointers
b, d = d, b
c = d

# Skip to the next element
# as it is already in order
d = d.next

# If b and d have some non-zero
# number of nodes in between them
else:

# Case 3: b is the head
# of the linked list
if b == head:

# Swap b.next and d.next
r = b.next
b.next = d.next
d.next = r
c.next = b

# Swap b and d pointers
b, d = d, b
c = d

# Skip to the next element
# as it is already in order
d = d.next

# Update the head
head = b

# Case 4: b is not the head
# of the linked list
else:

# Swap b.next and d.next
r = b.next
b.next = d.next
d.next = r
c.next = b
a.next = d

# Swap b and d pointers
b, d = d, b
c = d

# Skip to the next element
# as it is already in order
d = d.next

else:

# Update c and skip to the next element
# as it is already in order
c = d
d = d.next

a = b
b = b.next

return head

# Function to print the list
def printList(head):

while head:
print(head.data, end = ” “)
head = head.next

# Driver Code
if __name__ == “__main__”:

head = Node(5)
head.next = Node(4)
head.next.next = Node(3)

head = selectionSort(head)

printList(head)

# This code is contributed
# by Rituraj Jain

Output:

3 4 5


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