Java Program For Segregating Even And Odd Nodes In A Linked List

Given a Linked List of integers, write a function to modify the linked list such that all even numbers appear before all the odd numbers in the modified linked list. Also, keep the order of even and odd numbers same.
Examples:

Input: 17->15->8->12->10->5->4->1->7->6->NULL
Output: 8->12->10->4->6->17->15->5->1->7->NULL

Input: 8->12->10->5->4->1->6->NULL
Output: 8->12->10->4->6->5->1->NULL

// If all numbers are even then do not change the list
Input: 8->12->10->NULL
Output: 8->12->10->NULL

// If all numbers are odd then do not change the list
Input: 1->3->5->7->NULL
Output: 1->3->5->7->NULL

Method 1:
The idea is to get pointer to the last node of list. And then traverse the list starting from the head node and move the odd valued nodes from their current position to end of the list.
Thanks to blunderboy for suggesting this method. 
Algorithm: 

1. Get pointer to the last node.
2. Move all the odd nodes to the end.
   • Consider all odd nodes before the first even node and move them to end.
   • Change the head pointer to point to the first even node.
   • Consider all odd nodes after the first even node and move them to the end.

Java

// Java program to segregate even and
// odd nodes in a Linked List
class LinkedList
{
    // Head of list
    Node head;

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

    void segregateEvenOdd()
    {
        Node end = head;
        Node prev = null;
        Node curr = head;

        // Get pointer to last Node
        while (end.next != null)
            end = end.next;

        Node new_end = end;

        // Consider all odd nodes before
        // getting first even node
        while (curr.data % 2 !=0 &&
               curr != end)
        {
            new_end.next = curr;
            curr = curr.next;
            new_end.next.next = null;
            new_end = new_end.next;
        }

        // Do following steps only if
        // there is an even node
        if (curr.data % 2 == 0)
        {
            head = curr;

            // Now curr points to first even node
            while (curr != end)
            {
                if (curr.data % 2 == 0)
                {
                    prev = curr;
                    curr = curr.next;
                }
                else
                {
                    /* Break the link between prev
                       and curr*/
                    prev.next = curr.next;

                    // Make next of curr as null
                    curr.next = null;

                    // Move curr to end
                    new_end.next = curr;

                    // Make curr as new end of list
                    new_end = curr;

                    // Update curr pointer
                    curr = prev.next;
                }
            }
        }

        /* We have to set prev before
           executing rest of this code */
        else prev = curr;

        if (new_end != end &&
            end.data %2 != 0)
        {
            prev.next = end.next;
            end.next = null;
            new_end.next = end;
        }
    }

    /*  Given a reference (pointer to pointer)
        to the head of a list and an int, push
        a new node on the front of the list. */
    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;
    }

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

    // Driver code
    public static void main(String args[])
    {
        LinkedList llist = new LinkedList();
        llist.push(11);
        llist.push(10);
        llist.push(8);
        llist.push(6);
        llist.push(4);
        llist.push(2);
        llist.push(0);
        System.out.println(
               "Original Linked List");
        llist.printList();

        llist.segregateEvenOdd();

        System.out.println(
               "Modified Linked List");
        llist.printList();
    }
}
// This code is contributed by Rajat Mishra

Output

Original Linked List
0 2 4 6 8 10 11 
Modified Linked List
0 2 4 6 8 10 11 

Time complexity: O(n)
Auxiliary space: O(1)

Method 2:
The idea is to split the linked list into two: one containing all even nodes and other containing all odd nodes. And finally, attach the odd node linked list after the even node linked list. 
To split the Linked List, traverse the original Linked List and move all odd nodes to a separate Linked List of all odd nodes. At the end of the loop, the original list will have all the even nodes and the odd node list will have all the odd nodes. To keep the ordering of all nodes same, we must insert all the odd nodes at the end of the odd node list. And to do that in constant time, we must keep track of last pointer in the odd node list.

Java

// Java program to segregate even and
// odd nodes in a Linked List
import java.io.*;

class LinkedList
{
    // Head of list
    Node head;

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

    public void segregateEvenOdd()
    {
        Node evenStart = null;
        Node evenEnd = null;
        Node oddStart = null;
        Node oddEnd = null;
        Node currentNode = head;

        while(currentNode != null)
        {
            int element = currentNode.data;

            if(element % 2 == 0)
            {
                if(evenStart == null)
                {
                    evenStart = currentNode;
                    evenEnd = evenStart;
                }
                else
                {
                    evenEnd.next = currentNode;
                    evenEnd = evenEnd.next;
                }

            }
            else
            {
                if(oddStart == null)
                {
                    oddStart = currentNode;
                    oddEnd = oddStart;
                }
                else
                {
                    oddEnd.next = currentNode;
                    oddEnd = oddEnd.next;
                }
            }

            // Move head pointer one step
            // in forward direction
            currentNode = currentNode.next;
        }

        if(oddStart == null ||
           evenStart == null)
        {
            return;
        }

        evenEnd.next = oddStart;
        oddEnd.next = null;
        head=evenStart;
    }

    /*  Given a reference (pointer to pointer)
        to the head of a list and an int, push
        a new node on the front of the list. */
    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;
    }

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

    // Driver code
    public static void main(String args[])
    {
        LinkedList llist = new LinkedList();
        llist.push(11);
        llist.push(10);
        llist.push(9);
        llist.push(6);
        llist.push(4);
        llist.push(1);
        llist.push(0);
        System.out.println(
               "Original Linked List");
        llist.printList();

        llist.segregateEvenOdd();

        System.out.println(
               "Modified Linked List");
        llist.printList();
    }
}

Output

Original Linked List
0 1 4 6 9 10 11 
Modified Linked List
0 4 6 10 1 9 11 

Time complexity: O(n)
Auxiliary space: O(1) because using constant space

Please refer complete article on Segregate even and odd nodes in a Linked List for more details!