Monday, November 4, 2019

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

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 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 bankAfter 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 who 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 "null" element, So the first answer would most of the times 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 hold reference of the head, a pointer which 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 on Pluralsight.

2 example to find length of singly linked list in Java





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 to 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 singly linked list using recursion in Java:



2. Recursive Solution:

This is a much more concise then 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.


Further Learning
Data Structures and Algorithms: Deep Dive Using Java
Algorithms and Data Structures - Part 1 and 2
Data Structures in Java: An Interview Refresher
Cracking the Coding Interview - 189 Questions and Solutions


Related Data Structure and Algorithm Interview Questions from Javarevisited Blog
  • How to implement a linked list using generics in Java? (solution)
  • How to reverse a singly linked list in Java? (solution)
  • How to find the middle element of the linked list using a single pass? (solution)
  • How to find the 3rd element from the end of a linked list in Java? (solution)
  • Top 15 Data Structure and Algorithm Interview Questions (see here)
  • Top 20 String coding interview questions (see here)
  • 133 core Java interview questions of the last 5 years (see here)
  • Top 30 Array Coding Interview Questions with Answers (see here)
  • Top 30 linked list coding interview questions (see here)
  • Top 50 Java Programs from Coding Interviews (see here)
  • 5 Free Data Structure and Algorithms Courses for Programmers (courses)
  • 10 Algorithms Books Every Programmer Should Read (books)
  • 50+ Data Structure and Algorithms Problems from Interviews (questions)
  • 10 Free Data Structure and Algorithm Courses for Programmers (courses)
  • 100+ Data Structure Coding Problems from Interviews (questions)

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 question 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 the Easy to Advanced Data Structures course on Udemy. It's authored by a Google Software Engineer and Algorithm expert, and it's completely free of cost.

P. P. S. - Are you ready for an Interview? Take TripleByte's quiz and go directly to the final round of interviews with top tech companies like Coursera, Adobe, Dropbox, Grammarly, Uber, Quora, Evernote, Twitch, and many more.


11 comments :

LinkedList said...

This example to find length of linked list can also be extended to solve problems like how do you find middle element of linked list in Single pass or how to you find nth element from last in linked list in one loop. key here is to use two pointer one incrementing one at a time while other incrementing two at a time.

Anonymous said...

Just wondering why the interviewer asks these kind of questions when we have an size() method to get the size?

-Durai

Anonymous said...

@Durai

Even i wonder why they ask this question.

@Linked list : Anyways thanks for sharing information

Aditya Chaudhary said...

Java LinkedList is doubly Linked List.

Pancham said...

If Java LinkedList is doubly Linked List, then it should contaion two null elements, one in the begining and other at the end. Please correct if I am wrong

Anonymous said...

Java LinkedList is indeed Doubly linked list implementation of List and Deque interface in Java, as stated by Java doc. The do have two pointers first and last to point at both end of linked list.

Anonymous said...

Can you write a generic implementation of Singly linked list in Java?

Joyti said...

This question was asked to me as well, but they also asked me to find the complexity of both iterative and recursive solution. I said that if whether you use two pointer or one pointer approach you are checking almost each element, which means it's O(n) on times. Then he said, can you optimize the solution to reduce the time atleast to logarithimic or constant time. This question I couldn't answer because there is no other way to do this until you keep a pointer pointing to keep track of size. If anyone knows the aswer or trick, let me know please

Hengame Hoseini said...

why recursive method doesn't work?
I tried it for head as well, but it keep returns 0 instead of number of nodes....

Javin Paul said...

@Anonymous, you can find a generic implementation of a Singly linked list in Java here.

Anonymous said...

Does this program is also the solution of question "How to find length of linked list in one pass"? I am not sure, please clarify.

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