How do you find length of a Singly Linked list using Loop and Recursion? Example

Hello guys, here is one of the classical programming questions how do you find the length of a linked list using recursion and without recursion. This is not about the LinkedList class of Java API but the linked list data structure, which is made of nodes that contain both data and the address of the next node. In order to calculate the length, you need to count all nodes in the linked list. Since it's a singly linked list you can only move in one direction from the head to tail.  This coding problem was asked to me on my first interview with a multinational Investment bank. After that, this question has been asked to me on several occasions in other Programming Job Interviews as well.

What makes this question interesting is that Java developers are not that great with the data structure as compared to a C++ developer which is evident because of the fundamental difference between these two languages.

C++ is more of a system programming language, while Java is more on application programming. Also, a rich set of Java API allows the programmer to skip this kind of basic programming technique.

By the way, if you are preparing for programming job interviews and looking for more coding or algorithm-based questions like an array, string, or other data structures, you can always search this blog. I have shared a lot of problems in this blog over the years.

If you like to read books, then you can also take a look at Cracking the Coding Interviews, which contains 189 Programming Questions and Solutions. One of the best collections of coding questions for beginners, intermediate, and experienced programmers.

It contains questions from both fundamental and advanced data structures like a doubly-linked list, binary trees, self-balanced trees, and binary heaps. Btw, if you are not familiar with essential data structure, then  I suggest you join an excellent course like Data Structures and Algorithms: Deep Dive Using Java on Udemy; it's one of the best routes to learn and master data structure and Algorithms. Even if you know data structure, this can be used to further strengthen your knowledge.

2 Ways to find the length of the linked list in Java

Anyway let's come back to the question, everybody knows that in the singly linked list is the last element will point to the "null" element, So the first answer would most of the time be "I will use a counter and increment till it reaches the end of the element."

Once you reach the end of the linked list, the value of the counter would be equal to the total number of elements encountered, i.e. length of the elements.

Here is the sample code to implement this algorithm, you can use the linked list implementation given in this article for understanding the purpose. We are assuming the linked list class holds the reference of the head, a pointer that points to the first node in the linked list.

To learn more about the linked list data structure, you can pick an excellent course on data structure and algorithms like Algorithms and Data Structures - Part 1 and 2 courses on Pluralsight.

1. Iterative Solution

In this solution, we are just going through the entire linked list, one node at a time, until we reach the null, which is the end of the linked list.

public int length(){
 int count=0;
 Node current = this.head;

 while(current != null){
  count++;
  current=current.next()
 }
 return count;
}

If you answer this question without any difficulty, the most interviewer will ask you to write a "recursive" solution for this problem, just to check how you deal with recursion if your first answer would have been recursive they will ask you to write an "iterative solution" as shown above.

Btw, if you are preparing for a Programming job interview, it's essential to refresh data structure and algorithms before interviews.  I suggest you check out the Data Structures in Java: An Interview Refresher course on Educative to do well on your interview and practice more linked list-based coding questions.

And now, here is how to find the length of a singly linked list using recursion in Java:

2. Recursive Solution:

This is a much more concise than Iterative solution, but sometimes it's not easy to come up with the answer if you are not familiar with Recursion, but if you do, then it's straightforward.

public int length(Node current){
 if(current == null){ //base case
   return 0;
 }
 return 1+length(current.next());
}

You can see that we have used the fact that the last node will point to null to terminate the recursion. This is called the base case. It's essential to identify a base case while coding a recursive solution, without a base case, your program will never terminate and result in StackOverFlowError.



Related Data Structure and Algorithm Interview Questions from Javarevisited Blog

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Thanks for reading this article so far. If you like this article, then please share it with your friends and colleagues. If you have any questions or doubt then please let us know and I'll try to find an answer for you. As always suggestions, comments, innovative and better answers are most welcome.

P. S. - If you are looking for some Free Algorithms courses to improve your understanding of Data Structure and Algorithms, then you should also check these free data structure and algorithms courses from Coursera, Educative, and Udemy. It's authored by a Google Software Engineer and Algorithm expert, and it's completely free of cost.