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Reversing first k nodes of a Linked List


In this post, we will discuss a method to reverse k nodes from the starting of the linked list. In one of my earlier post, we have already discussed methods to reverse a given linked list. Here We will create a method that accepts the head of the linked list and Number k that indicates the number of elements to be reversed. To do this we will be using a simple method to reverse the linked list with an additional condition in the while loop where decrease k after each iteration. Have a look at my previous post on Linked List in c++ to see how to reverse a linked list.

Let’s directly look at the algorithm and code to understand it better

Algorithm:
  1. In the above code, we use 3 pointers current, previous and next. 
  2. Initially, the current node points to the head node, the previous and next nodes are Null 
  3. Then we use a while loop to traverse through the linked list and also decrease k 
  4. In each iteration, we do the following 
    • assign next pointer to current node's link field 
    • update current nodes link field to point to the previous node 
    • update the previous node to point to the current node 
    • when k is 0 then previous node points to the starting of the linked list so we assign it to the head node 
  5. We also need to join the link between the current node and the next node 
  6. Return head node

 Suggestion: To understand the algorithm and its working better try doing a dry run on the paper by considering a simple linked list. 


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