Java program to reverse linked list in pairs

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Java Linked List Interview Programs:

If you want to practice data structure and algorithm programs, you can go through data structure and algorithm interview questions.

This is one of popular interview question. In this post, we will see how to reverse linked list in pairs.

Java Linked List Interview Programs:

There can be two solution for reversing linked list in pairs

Iterative
Recursive

Iterative:

Logic for this would be:

public static Node reverseLinkedListInPairItr(Node head) {

    Node current=head;
    Node temp=null;
    Node newHead =null;
    while (current != null && current.next != null) {

        if (temp != null) {
            // This is important step
            temp.next.next = current.next;
        }
        temp=current.next;
        current.next=temp.next;
        temp.next=current;

        if (newHead == null)
            newHead = temp;
        current=current.next;

    }
    return newHead;
}

public static Node reverseLinkedListInPairItr(Node head) {

Node current=head;

Node temp=null;

Node newHead =null;

while (current != null && current.next != null) {

if (temp != null) {

// This is important step

temp.next.next = current.next;

}

temp=current.next;

current.next=temp.next;

temp.next=current;

if (newHead == null)

newHead = temp;

current=current.next;

}

return newHead;

}

Explanation:

Lets understand with the help of simple example:
Lets say linkedlist is 5-> 6 -> 7 -> 1
If you look closely, below steps are just reversing link to 6->5->7->1 in first iteration (Swapping node 6 and 5) and then advance to next pair (Node 7 and 1)

// Node 6 is temp node
temp=current.next;
// putting link between 5->7
current.next=temp.next;
// putting link between 6->5
temp.next=current;
// 6 becomes head node here

// Node 6 is temp node

temp=current.next;

// putting link between 5->7

current.next=temp.next;

// putting link between 6->5

temp.next=current;

// 6 becomes head node here

You must be wondering why we have put below code:

if (temp != null) {
      // This is important step
      temp.next.next = current.next;
}

if (temp != null) {

// This is important step

temp.next.next = current.next;

}

before swapping two nodes , we need to link nodes properly. When we are swapping 7 and 1 , link between 5->7 will be broken and we need to connect node 5 to node 1.
As per above code , temp will be null in first iteration(swapping node 6 and 5) but when current node is 7, temp will be 6, here we are linking nodes as below
6->5->1->7

Java program for this will be :

package org.arpit.java2blog;

public class ReverseLinkedListInPair{

    private Node head;

    private static class Node {
        private int value;
        private Node next;

        Node(int value) {
            this.value = value;

        }
    }

    public void addToTheLast(Node node) {

        if (head == null) {
            head = node;
        } else {
            Node temp = head;
            while (temp.next != null)
                temp = temp.next;

            temp.next = node;
        }
    }

    public void printList(Node head) {
        Node temp = head;
        while (temp != null) {
            System.out.format("%d ", temp.value);
            temp = temp.next;
        }
        System.out.println();
    }

    // Reverse linked list in pair
    public static Node reverseLinkedListInPairs(Node head) {

        Node current=head;
        Node temp=null;
        Node newHead =null;
        while (current != null && current.next != null) {

            if (temp != null) {
                // This is important step
                temp.next.next = current.next;
            }
            temp=current.next;
            current.next=temp.next;
            temp.next=current;

            if (newHead == null)
                newHead = temp;
            current=current.next;

        }
        return newHead;
    }

    public static void main(String[] args) {
        ReverseLinkedListInPair list = new ReverseLinkedListInPair();
        // Creating a linked list
        Node head=new Node(5);
        list.addToTheLast(head);
        list.addToTheLast(new Node(6));
        list.addToTheLast(new Node(7));
        list.addToTheLast(new Node(1));
        list.addToTheLast(new Node(2));
        list.addToTheLast(new Node(8));

        list.printList(head);
        //Reversing LinkedList in pairs
        Node result=reverseLinkedListInPairs(head);
        System.out.println("After reversing in pair");
        list.printList(result);
    }
}

package org.arpit.java2blog;

public class ReverseLinkedListInPair{

private Node head;

private static class Node {

private int value;

private Node next;

Node(int value) {

this.value = value;

}

public void addToTheLast(Node node) {

if (head == null) {

head = node;

} else {

Node temp = head;

while (temp.next != null)

temp = temp.next;

temp.next = node;

}

public void printList(Node head) {

Node temp = head;

while (temp != null) {

System.out.format("%d ", temp.value);

temp = temp.next;

}

System.out.println();

}

// Reverse linked list in pair

public static Node reverseLinkedListInPairs(Node head) {

Node current=head;

Node temp=null;

Node newHead =null;

while (current != null && current.next != null) {

if (temp != null) {

// This is important step

temp.next.next = current.next;

}

temp=current.next;

current.next=temp.next;

temp.next=current;

if (newHead == null)

newHead = temp;

current=current.next;

}

return newHead;

}

public static void main(String[] args) {

ReverseLinkedListInPair list = new ReverseLinkedListInPair();

// Creating a linked list

Node head=new Node(5);

list.addToTheLast(head);

list.addToTheLast(new Node(6));

list.addToTheLast(new Node(7));

list.addToTheLast(new Node(1));

list.addToTheLast(new Node(2));

list.addToTheLast(new Node(8));

list.printList(head);

//Reversing LinkedList in pairs

Node result=reverseLinkedListInPairs(head);

System.out.println("After reversing in pair");

list.printList(result);

}

Run above program, you will get following output:

5 6 7 1 2 8
After reversing
6 5 1 7 8 2

5 6 7 1 2 8

After reversing

6 5 1 7 8 2

Recursive:

Base case: Base case for this would be either node is null or node.next is null

For recursive solution, replace reverseLinkedListInPairs of above program to below function

 public static Node reverseLinkedListInPairs(Node head) {
      if (head == null || head.next == null) {
          return head;
      }
  // Lets take example of 5->6->7
  // Here head node is 5
  // Storing 6 in temp node, it will become our new head
     Node temp=head.next;
  // Putting link between 5->7
     head.next=temp.next;
  // putting link between 6->5
     temp.next=head;
  // recursively calling the function for node 7
     head.next=reverseLinkedListInPairRec(head.next);
  // returning new head
     return temp;
  }

public static Node reverseLinkedListInPairs(Node head) {

if (head == null || head.next == null) {

return head;

}

// Lets take example of 5->6->7

// Here head node is 5

// Storing 6 in temp node, it will become our new head

Node temp=head.next;

// Putting link between 5->7

head.next=temp.next;

// putting link between 6->5

temp.next=head;

// recursively calling the function for node 7

head.next=reverseLinkedListInPairRec(head.next);

// returning new head

return temp;

}

Output of this program will be same as above program.

Please go through java interview programs for more such programs.

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Java program to reverse linked list in pairs

Java Linked List Interview Programs:

Iterative:

Explanation:

Recursive:

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Java Linked List Interview Programs:

Iterative:

Explanation:

Recursive:

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