REVERSE A LINKED LIST HOW TO
REVERSE A LINKED LIST CODE

It also provides you with a way of thinking and approaching any twisted Linked List question in your coding interviews.

It is an important first step in your coding interview prep to make yourself comfortable handling pointers and references. While I know most companies don't test you on Linked List questions these days. The steps below describe how an iterative algorithm works: Three-pointers must be initialized, which are called ptrA, ptrB and ptrC. Once you solve the above practice problems, you should be pretty comfortable with traversing around a singly linked list. A linked list can be reversed in java using two algorithms. The only tricky part is how you manage the pointers to perform the stitching of the reversed sublists.Ģ.2 Reverse Nodes in Alternate k-Groups - I didn't find this on Leetcode, but the only change in this problem is you reverse every alternate k-group of nodes. Reverse Nodes in k-Group - Conceptually it is the same as the problems we discussed in this post. after you are done, get the pointer of the first node after your original list to point at a reversed list. ( (original list length - current node index) 2 + 1 ) size of node. Add Two Numbers II - Then look at this problem and think about how you can apply the reversal pattern hereĢ.1. copy the contents of current node into the node having pointer advancing by. Add Two Numbers - First practice this problemġ.2. You can apply the reversing of the linked list pattern to the following problems on Leetcode:ġ.1.

REVERSE A LINKED LIST CODE

Here's a visual representation of the linked list reversal algorithm we discussed above: Visual flow for reversing a Singly Linked List Code: public Node reverse(ListNode head) Code to reverse a sub-list in Java Applying the Pattern. Because prev will now be pointing to the last node of the original list, and it is now the head of the newly reversed linked list.

Keep iterating until curr reaches the end of the list and there are no more nodes to be reversed. So, now we can move the curr pointer to fwd, which is the new node we need to process for reversing.ģ.4. Because in step 3.1, we had already placed the fwd pointer ahead of curr. next) to the prev pointer.īasically, starting the reversal process for curr node.ģ.3. Make the curr node's next pointer ( curr. Place the fwd pointer on the node just after the Current Pointer ( curr).ģ.2.

Initially, the prev and next pointers won't be referencing any nodeģ.1.

Place the curr node at the head of the list.

To reverse a linked list iteratively and in place, we need 3 new pointers: Previous (prev), Current (curr), and Forward (fwd) pointer.

Below is an interesting approach that uses only two pointers. In iterative method we had used 3 pointers prev, cur and next.

REVERSE A LINKED LIST HOW TO

As the DLLNode maintains gives us two pointers, one pointing to the previous node and one pointing to the next node in the list. We have seen how to reverse a linked list in article Reverse a linked list. Our only way of traversing backward is to reverse the linked list.Ĭompared to Singly Linked List, traversing backward would have been easier if we had a Doubly Linked List. After moving, to go back to the previous node we won't have any reference information in our current Node data structure. This means we can only move forward from one node to the other as we always only maintain a pointer to only the next node in the list. We are using the custom linked list implementation that we implemented in our last example.We learned in the Basics of Linked Lists that traversal in a Singly Linked list is uni-directional. Move previous and current pointer one step aheadīelow is the Java code implementation. Point current node next pointer to previous nodeĥ. Store current->next pointer to next nodeĤ. current node is null and repeat step 3, 4 and 5ģ. Iterate the list until the tail node is reached i.e. Initialize three pointers prev as NULL, current as head, and next as NULL.Ģ. The iterative way follows the below algorithm.ġ. Assuming the current pointer starts with the head pointer and the previous pointer points to a NULL reference, after every iteration, the current pointer starts pointing to the previous pointer and then move previous and current pointer one step ahead. The iterative way requires 3 different pointers to reverse a given linked list and in each iteration the current node next pointer points back to the previous node. Given below a linked list and its corresponding reversed list. Reversing a linked list means reversing the pointer between the nodes but not the actual node data. In this article, we will discuss 2 different ways to reverse a given linked list using an iterative and recursive way in Java.