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Finding kth element from end of Linked List


In one of my previous post I discussed a method to find the middle element of the linked list in single traversal using 2 pointers. In this post We will see a similar approach to find the kth element from the end of the linked list.

Finding the kth element from the end of the linked list is one of the famous interview questions and is asked a lot.

In order to find the kth element from the end of the linked list we use 2 pointers. Let the 2 pointers be first and second. now we initialize them to point at the head node of the linked list and move the second pointer to the kth node. After that we start traversing the linked list using both the pointer and keep moving them forward until the first pointer reaches the end of the linked list. as the difference between the position of the pointers is k places when the first node points the last node then the second pointer will point to the kth element from the end. So we can simply return the second pointer or the contents of the the node pointed by the second pointer.

The idea is to have a difference of k places between the first and the second pointers. using this method, we can find the kth element from the end in a single
traversal and we do not need to traverse the linked list twice.

So time complexity of this approach is o(n) and space complexity is o(1).

Algorithm:

  1. Initialize 2 pointers first and second to head node
  2. Move node first to the kth node from the start
  3. Move both first and second node through the list till first node reaches the last node
  4. At this point the second node points to the kth node from the end
  5. Return the contents of the second node

Kth element from the end image
Code : 

C++ Program

Sample input and output to check the program



You might also be interested in 

Singly Linked List
Double Linked List
Linked List in Python
Stack Implementation using Linked List
Queue Implementation using Linked List
Vigenere Cipher Encryption
Check for Anagram Strings


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