How do you reverse a singly linked list without recursion in Java?

Implementing Your Own Linked List on Interviews

Since using existing Java classes is now allowed on Programming Job interviews, you need to create your own to write code. For this example, I have created our own singly linked list class. Similar to java.util.LinkedListalso contains a nested static class Node, which represents a node in the linked list.

This class contains an integer attribute to hold the data part and another Node reference to point to the next one in the list. If you want to create a Generic linked list, you should replace int with T, a generic type, as shown here.

In order to demonstrate that our reverse method is working, we will not only have to create a linked list but also need to populate the linked list. In order to populate, you need to implement the add[] method on the singly linked list.

You have two choices, either add the element at the head or at the tail, adding an element to the head is easy as it doesn't require a traversal till the end but if you want to create a list that contains elements in the order they are added then we need to add nodes at the end of the linked list.

I have also created a print[] method to print all nodes of the singly linked list, separated by space. This method is very useful to demonstrate that our reverse method is actually working or not, as you can print the linked list before and after reversal.

If you struggle with implementing essential data structures like a linked list, binary tree, the hash table in your own code on any programming language like Java then I suggest you joinAlgorithms and Data Structures - Part 1 and 2courses on Pluralsight. They will not only help you to write your data structure but also how to calculate time and space complexity.

Algorithm –reverse single linked list in java [iterative algorithm]

1. head variable denotes head of single linked list.
2. Take couple of temporary variables
   • next = null
   • prev = null
3. next = head.next [Step 1]
   • save reference of subsequent node in next variable
4. head.next = prev[node reversed] [Step 2]
5. prev = head [Step 3]
6. head = next [head points to second node] [Step 4]

Nowapply the algorithm to reverse the linked list. Once we apply the above algorithm to complete linked list, we will get the reverse linked list as shown in Fig 3

Time complexity of algorithm is O[n].

“reverse linked list without recursion” Code Answer

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C

1. Introduction

In this tutorial, we'll implement two linked list reversal algorithms in Java.

Java Program to Reverse a singly linked list using recursion and Iteration

A linked list is a data structure which contains nodes, every node keep data and pointer to the next node. This way linked list grows and can store as many elements as much memory allows it. It's not like an array that requires a contiguous chunk of memory because here node can be stored at any memory location.

This structure means, adding and removing elements in a linked list is easy but searching an element is expensive because you need to traverse the entire list to find the element. It doesn't help even if you know that element is the 5th node or 6th node because you cannot access them by index like an array.

This is the biggest difference between an array and a linked list data structure. In the array, searching the index is O[1] operation but in linked list searching is O[n] operation.


It divides the list into two parts first node and rest of the list,and then link rest to head in reverse order. It then recursively applies the same division until it reaches the last node, at that point whole linked list, is reversed.

Coming back to our code which represents a singly linked list in Java [see the next section], with limited operations. I have already removed some non-relevant code for performing different operations on a linked list likechecking if the linked list is cyclic or not, inserting an element at the middle, and removing the element. Since we don't need this code for reversing a linked list, I have simply deleted them for now.

This class is similar to theSinglyLinkedListclass, which we have seen in how to implement a linked list in Java using generics [seehere], with two more methods for reversing linked list using iteration and recursion.

The reverseRecursively[]methodreverses the linked list using recursion. It uses the call stack to store data, and once we reached tail, which becomes the new head for the reversed linked list, it starts adding nodes in reverse order. Look at some comments around those methods, which will make you understand the algorithm of reversing the linked list better.
ThereverseIteratively[]method reverses the linked list using the three-pointers approach and using loops, that's why it is called an iterative solution. It traverses through the linked list and adding nodes at the beginning of the singly linked list in each iteration. It uses three reference variables [pointers] to keep track of previous, current, and next nodes.

Btw, If you are not very familiar with a linked list data structure or want to learn more about linked list data structure, you should first read a good course on data structure and algorithm likeAlgorithms and Data Structures - Part 1 and 2on Pluralsight, one of the best course to learn data structure and algorithms.