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

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.

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## 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.

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## 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;

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.

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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.

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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

Unknown said...

Pancham Goyal 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

Unknown 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.

Anonymous said...