Sort linked list which is already sorted on absolute values

Given a linked list that is sorted based on absolute values. Sort the list based on actual values.

Examples:

Input :  1 -> -10 
output: -10 -> 1

Input : 1 -> -2 -> -3 -> 4 -> -5 
output: -5 -> -3 -> -2 -> 1 -> 4 

Input : -5 -> -10 
Output: -10 -> -5

Input : 5 -> 10 
output: 5 -> 10

Source : Amazon Interview

A simple solution is to traverse the linked list from beginning to end. For every visited node, check if it is out of order. If it is, remove it from its current position and insert it at the correct position. This is the implementation of insertion sort for linked list and the time complexity of this solution is O(n*n).

A better solution is to sort the linked list using merge sort. Time complexity of this solution is O(n Log n).

An efficient solution can work in O(n) time. An important observation is, all negative elements are present in reverse order. So we traverse the list, whenever we find an element that is out of order, we move it to the front of the linked list. 

Below is the implementation of the above idea. 

C++

// C++ program to sort a linked list, already
// sorted by absolute values
#include <bits/stdc++.h>
using namespace std;

// Linked List Node
struct Node
{
    Node* next;
    int data;
};

// Utility function to insert a node at the
// beginning
void push(Node** head, int data)
{
    Node* newNode = new Node;
    newNode->next = (*head);
    newNode->data = data;
    (*head) = newNode;
}

// Utility function to print a linked list
void printList(Node* head)
{
    while (head != NULL)
    {
        cout << head->data;
        if (head->next != NULL)
            cout << " -> ";
        head = head->next;
    }
    cout<<endl;
}

// To sort a linked list by actual values.
// The list is assumed to be sorted by absolute
// values.
void sortList(Node** head)
{
    // Initialize previous and current nodes
    Node* prev = (*head);
    Node* curr = (*head)->next;

    // Traverse list
    while (curr != NULL)
    {
        // If curr is smaller than prev, then
        // it must be moved to head
        if (curr->data < prev->data)
        {
            // Detach curr from linked list
            prev->next = curr->next;

            // Move current node to beginning
            curr->next = (*head);
            (*head) = curr;

            // Update current
            curr = prev;
        }

        // Nothing to do if current element
        // is at right place
        else
            prev = curr;

        // Move current
        curr = curr->next;
    }
}

// Driver code
int main()
{
    Node* head = NULL;
    push(&head, -5);
    push(&head, 5);
    push(&head, 4);
    push(&head, 3);
    push(&head, -2);
    push(&head, 1);
    push(&head, 0);

    cout << "Original list :\n";
    printList(head);

    sortList(&head);

    cout << "\nSorted list :\n";
    printList(head);

    return 0;
}

Java

// Java program to sort a linked list, already
// sorted by absolute values
class SortList
{
    static Node head;  // head of list

    /* Linked list Node*/
    static class Node
    {
        int data;
        Node next;
        Node(int d) {data = d; next = null; }
    }

    // To sort a linked list by actual values.
        // The list is assumed to be sorted by absolute
        // values.
    Node sortedList(Node head)
    {
        // Initialize previous and current nodes
        Node prev = head;
        Node curr = head.next;

        // Traverse list
        while(curr != null)
        {
            // If curr is smaller than prev, then
                        // it must be moved to head
            if(curr.data < prev.data)
            {
                // Detach curr from linked list
                prev.next = curr.next;

                // Move current node to beginning
                curr.next = head;
                head = curr;

                // Update current
                curr = prev;
            }

            // Nothing to do if current element
                        // is at right place
            else
            prev = curr;

            // Move current
            curr = curr.next;
        }
        return head;
    }

    /* Inserts a new Node at front of the list. */
    public void push(int new_data)
    {
        /* 1 & 2: Allocate the Node &
                  Put in the data*/
        Node new_node = new Node(new_data);

        /* 3. Make next of new Node as head */
        new_node.next = head;

        /* 4. Move the head to point to new Node */
        head = new_node;
    }

    /* Function to print linked list */
    void printList(Node head)
    {
        Node temp = head;
        while (temp != null)
        {
           System.out.print(temp.data+" ");
           temp = temp.next;
        }
        System.out.println();
    }

    /* Driver program to test above functions */
    public static void main(String args[])
    {
        SortList llist = new SortList();

        /* Constructed Linked List is 1->2->3->4->5->6->
           7->8->8->9->null */
        llist.push(-5);
        llist.push(5);
        llist.push(4);
        llist.push(3);
        llist.push(-2);
        llist.push(1);
        llist.push(0);

        System.out.println("Original List :");
        llist.printList(llist.head);

        llist.head = llist.sortedList(head);

        System.out.println("Sorted list :");
        llist.printList(llist.head);
    }
}

// This code has been contributed by Amit Khandelwal(Amit Khandelwal 1).

Python3

# Python3 program to sort a linked list,
# already sorted by absolute values

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

class SortList:
    def __init__(self):
        self.head = None

    # To sort a linked list by actual values.
    # The list is assumed to be sorted by
    # absolute values.
    def sortedList(self, head):

        # Initialize previous and
        # current nodes
        prev = self.head
        curr = self.head.next

        # Traverse list
        while(curr != None):

            # If curr is smaller than prev,
            # then it must be moved to head
            if(curr.data < prev.data):

                # Detach curr from linked list
                prev.next = curr.next

                # Move current node to beginning
                curr.next = self.head
                self.head = curr

                # Update current
                curr = prev

            # Nothing to do if current element
            # is at right place
            else:
                prev = curr

            # Move current
            curr = curr.next
        return self.head

    # Inserts a new Node at front of the list
    def push(self, new_data):

        # 1 & 2: Allocate the Node &
        #        Put in the data
        new_node = Node(new_data)

        # 3. Make next of new Node as head
        new_node.next = self.head

        # 4. Move the head to point to new Node
        self.head = new_node

    # Function to print linked list
    def printList(self, head):
        temp = head
        while (temp != None):
            print(temp.data, end = " ")
            temp = temp.next
        print()

# Driver Code
llist = SortList()

# Constructed Linked List is
# 1->2->3->4->5->6->7->8->8->9->null
llist.push(-5)
llist.push(5)
llist.push(4)
llist.push(3)
llist.push(-2)
llist.push(1)
llist.push(0)

print("Original List :")
llist.printList(llist.head)

start = llist.sortedList(llist.head)

print("Sorted list :")
llist.printList(start)

# This code is contributed by
# Prerna Saini

C#

// C# program to sort a linked list, already
// sorted by absolute values
using System;

public class SortList
{
    Node head; // head of list

    /* Linked list Node*/
    class Node
    {
        public int data;
        public Node next;
        public Node(int d)
        {
            data = d; next = null;
        }
    }

    // To sort a linked list by actual values.
    // The list is assumed to be sorted by absolute
    // values.
    Node sortedList(Node head)
    {
        // Initialize previous and current nodes
        Node prev = head;
        Node curr = head.next;

        // Traverse list
        while(curr != null)
        {
            // If curr is smaller than prev, then
            // it must be moved to head
            if(curr.data < prev.data)
            {
                // Detach curr from linked list
                prev.next = curr.next;

                // Move current node to beginning
                curr.next = head;
                head = curr;

                // Update current
                curr = prev;
            }

            // Nothing to do if current element
            // is at right place
            else
            prev = curr;

            // Move current
            curr = curr.next;
        }
        return head;
    }

    /* Inserts a new Node at front of the list. */
    public void push(int new_data)
    {
        /* 1 & 2: Allocate the Node &
                Put in the data*/
        Node new_node = new Node(new_data);

        /* 3. Make next of new Node as head */
        new_node.next = head;

        /* 4. Move the head to point to new Node */
        head = new_node;
    }

    /* Function to print linked list */
    void printList(Node head)
    {
        Node temp = head;
        while (temp != null)
        {
        Console.Write(temp.data + " ");
        temp = temp.next;
        }
        Console.WriteLine();
    }

    /* Driver code */
    public static void Main(String []args)
    {
        SortList llist = new SortList();

        /* Constructed Linked List is 1->2->3->
        4->5->6->7->8->8->9->null */
        llist.push(-5);
        llist.push(5);
        llist.push(4);
        llist.push(3);
        llist.push(-2);
        llist.push(1);
        llist.push(0);

        Console.WriteLine("Original List :");
        llist.printList(llist.head);

        llist.head = llist.sortedList(llist.head);

        Console.WriteLine("Sorted list :");
        llist.printList(llist.head);
    }
}

/* This code is contributed by 29AjayKumar */

Javascript

<script>

// Javascript program to sort a linked list, already
// sorted by absolute values

    var head; // head of list

    /* Linked list Node */
     class Node {
            constructor(d) {
                this.data = d;
                this.next = null;
            }
        }

    // To sort a linked list by actual values.
    // The list is assumed to be sorted by absolute
    // values.
    function sortedList(head) {
        // Initialize previous and current nodes
    var prev = head;
    var curr = head.next;

        // Traverse list
        while (curr != null) {
            // If curr is smaller than prev, then
            // it must be moved to head
            if (curr.data < prev.data) {
                // Detach curr from linked list
                prev.next = curr.next;

                // Move current node to beginning
                curr.next = head;
                head = curr;

                // Update current
                curr = prev;
            }

            // Nothing to do if current element
            // is at right place
            else
                prev = curr;

            // Move current
            curr = curr.next;
        }
        return head;
    }

    /* Inserts a new Node at front of the list. */
     function push(new_data) {
        /*
         * 1 & 2: Allocate the Node & Put in the data
         */
var new_node = new Node(new_data);

        /* 3. Make next of new Node as head */
        new_node.next = head;

        /* 4. Move the head to point to new Node */
        head = new_node;
    }

    /* Function to print linked list */
    function printList(head) {
var temp = head;
        while (temp != null) {
            document.write(temp.data + " ");
            temp = temp.next;
        }
        document.write("<br/>");
    }

    /* Driver program to test above functions */


        /*
          Constructed Linked List is
         1->2->3->4->5->6-> 7->8->8->9->null
         */
        push(-5);
        push(5);
        push(4);
        push(3);
        push(-2);
        push(1);
        push(0);

        document.write("Original List :<br/>");
        printList(head);

        head = sortedList(head);

        document.write("Sorted list :<br/>");
        printList(head);

// This code contributed by aashish1995

</script>

Output

Original list :
0 -> 1 -> -2 -> 3 -> 4 -> 5 -> -5

Sorted list :
-5 -> -2 -> 0 -> 1 -> 3 -> 4 -> 5

Time Complexity: O(N)
Auxiliary Space: O(1)

This article is contributed by Rahul Titare. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.