Reverse a LinkedList in Java
Last Updated :
21 Nov, 2022
Assuming you have gone through LinkedList in java and know about linked list. This post contains different examples for reversing a linked list which are given below: 1. By writing our own function(Using additional space): reverseLinkedList() method contains logic for reversing string objects in a linked list. This method takes a linked list as a parameter, traverses all the elements in reverse order and adds it to the newly created linked list. Algorithm: Step 1. Create a linked list with n elements Step 2. Create an empty linked list which will be used to store reversed elements Step 3. Start traversing the list from 'n' to '0' and store the elements in the newly created list. Step 4. The elements will be stored in the following order: n, n-1, n-2, ......0 Step 5. Return the list to the caller and print it
Example:
Step 1: LL: 1 -> 2 -> 3 -> 4 -> 5 where 'LL' is the linked list with n elements
Step 2: 'Rev' is an empty linked list
Step 3: Start traversing, the below passes are the intermediate steps while traversing
1st pass: Rev: 5
2nd pass: Rev: 5 -> 4
3rd pass: Rev: 5 -> 4 -> 3
4th pass: Rev: 5 -> 4 -> 3 -> 2
5th pass: Rev: 5 -> 4 -> 3 -> 2 -> 1
Step 4: nth element of 'LL' is stored in 0th position of 'Rev',
n-1 element of LL is stored in 1st position of Rev and so on......
Step 5: Return Rev: 5 -> 4 -> 3 -> 2 -> 1 to the calling function.
Java
// Java program for reversing linked list using additional space
import java.util.*;
public class LinkedListTest1 {
public static void main(String[] args)
{
// Declaring linkedlist without any initial size
LinkedList<String> linkedli = new LinkedList<String>();
// Appending elements at the end of the list
linkedli.add("Cherry");
linkedli.add("Chennai");
linkedli.add("Bullet");
System.out.print("Elements before reversing: " + linkedli);
linkedli = reverseLinkedList(linkedli);
System.out.print("\nElements after reversing: " + linkedli);
}
// Takes a linkedlist as a parameter and returns a reversed linkedlist
public static LinkedList<String> reverseLinkedList(LinkedList<String> llist)
{
LinkedList<String> revLinkedList = new LinkedList<String>();
for (int i = llist.size() - 1; i >= 0; i--) {
// Append the elements in reverse order
revLinkedList.add(llist.get(i));
}
// Return the reversed arraylist
return revLinkedList;
}
}
Time Complexity: O(n) Space Complexity: O(n) NOTE: As we are using additional memory space for storing all the reversed 'n' elements, the space complexity is O(n).
Output:Elements before reversing: [Cherry, Chennai, Bullet]
Elements after reversing: [Bullet, Chennai, Cherry]
2. By writing our own function(Without using additional space): In the previous example, a linked list is used additionally for storing all the reversed elements which takes more space. To avoid that, same linked list can be used for reversing. Algorithm: 1. Create a linked list with n elements 1. Run the loop for n/2 times where ‘n’ is the number of elements in the linkedlist. 2. In the first pass, Swap the first and nth element 3. In the second pass, Swap the second and (n-1)th element and so on till you reach the mid of the linked list. 4. Return the linked list after loop termination.
Example:
Input: 1 -> 2 -> 3 -> 4 -> 5
1st pass: (swap first and nth element)
5 -> 2 -> 3 -> 4 -> 1
2nd pass: (swap second and (n-1)th element)
5 -> 4 -> 3 -> 2 -> 1
3rd pass: (reached mid, Terminate loop)
5 -> 4 -> 3 -> 2 -> 1
Output: 5 -> 4 -> 3 -> 2 -> 1
Java
// Java program for reversing an arraylist without
// using any additional space
import java.util.*;
public class LinkedListTest2 {
public static void main(String[] args)
{
// Declaring linkedlist without any initial size
LinkedList<Integer> linkedli = new LinkedList<Integer>();
// Appending elements at the end of the list
linkedli.add(new Integer(1));
linkedli.add(new Integer(2));
linkedli.add(new Integer(3));
linkedli.add(new Integer(4));
linkedli.add(new Integer(5));
System.out.print("Elements before reversing: " + linkedli);
// Calling user defined function for reversing
linkedli = reverseLinkedList(linkedli);
System.out.print("\nElements after reversing: " + linkedli);
}
// Takes a linkedlist as a parameter and returns a reversed linkedlist
public static LinkedList<Integer> reverseLinkedList(LinkedList<Integer> llist)
{
for (int i = 0; i < llist.size() / 2; i++) {
Integer temp = llist.get(i);
llist.set(i, llist.get(llist.size() - i - 1));
llist.set(llist.size() - i - 1, temp);
}
// Return the reversed arraylist
return llist;
}
}
Time Complexity: O(n/2) Space Complexity: O(1)
Output:Elements before reversing: [1, 2, 3, 4, 5]
Elements after reversing: [5, 4, 3, 2, 1]
3. By using Collections class: Collections is a class in java.util package which contains various static methods for searching, sorting, reversing, finding max, min….etc. We can make use of the In-built Collections.reverse() method for reversing an linked list. It takes a list as an input parameter and returns the reversed list. NOTE: Collections.reverse() method uses the same algorithm as "By writing our own function(Without using additional space)"
Java
// Java program for reversing a linked list using
// In-built collections class
import java.util.*;
public class LinkedListTest3 {
public static void main(String[] args)
{
// Declaring linkedlist without any initial size
LinkedList<Integer> linkedli = new LinkedList<Integer>();
// Appending elements at the end of the list
linkedli.add(new Integer(1));
linkedli.add(new Integer(2));
linkedli.add(new Integer(3));
linkedli.add(new Integer(4));
linkedli.add(new Integer(5));
System.out.print("Elements before reversing: " + linkedli);
// Collections.reverse method takes a list as a
// parameter and returns the reversed list
Collections.reverse(linkedli);
System.out.print("\nElements after reversing: " + linkedli);
}
}
Time Complexity: O(n/2) Space Complexity: O(1)
Output:Elements before reversing: [1, 2, 3, 4, 5]
Elements after reversing: [5, 4, 3, 2, 1]
4.Reversing a linked list of user defined objects: An Employee class is created for creating user defined objects with employeeID, employeeName, departmentName as class variables which are initialized in the constructor. An linked list is created that takes only Employee(user defined) Objects. These objects are added to the linked list using add() method. The linked list is reversed using In-built reverse() method of Collections class. printElements() method is used to iterate through all the user defined objects in the linked list and print the employee ID, name and department name for every object.
Java
// Java program for reversing a linkedlist of user defined objects
import java.util.*;
class Employee {
int empID;
String empName;
String deptName;
// Constructor for initializing the class variables
public Employee(int empID, String empName, String deptName)
{
this.empID = empID;
this.empName = empName;
this.deptName = deptName;
}
}
public class LinkedListTest4 {
public static void main(String[] args)
{
// Declaring linkedList without any initial size
LinkedList<Employee> linkedli = new LinkedList<Employee>();
// Creating user defined objects
Employee emp1 = new Employee(123, "Cherry", "Fashionist");
Employee emp2 = new Employee(124, "muppala", "Development");
Employee emp3 = new Employee(125, "Bullet", "Police");
// Appending all the objects to linkedList
linkedli.add(emp1);
linkedli.add(emp2);
linkedli.add(emp3);
System.out.print("Elements before reversing: ");
printElements(linkedli);
// Collections.reverse method takes a list as a parameter
// and returns the reversed list
Collections.reverse(linkedli);
System.out.print("\nElements after reversing: ");
printElements(linkedli);
}
// Iterate through all the elements and print
public static void printElements(LinkedList<Employee> llist)
{
for (int i = 0; i < llist.size(); i++) {
System.out.print("\n EmpID:" + llist.get(i).empID + ", EmpName:"
+ llist.get(i).empName + ", Department:" + llist.get(i).deptName);
}
}
}
Time Complexity: O(n/2) Space Complexity: O(1)
Output:Elements before reversing:
EmpID:123, EmpName:Cherry, Department:Fashionist
EmpID:124, EmpName:muppala, Department:Development
EmpID:125, EmpName:Bullet, Department:Police
Elements after reversing:
EmpID:125, EmpName:Bullet, Department:Police
EmpID:124, EmpName:muppala, Department:Development
EmpID:123, EmpName:Cherry, Department:Fashionist
See reverse a linked list for reversing a user defined linked list.
Similar Reads
LinkedList in Java
Linked List is a part of the Collection framework present in java.util package. This class is an implementation of the LinkedList data structure which is a linear data structure where the elements are not stored in contiguous locations and every element is a separate object with a data part and addr
12 min read
ArrayList vs LinkedList in Java
An array is a collection of items stored at contiguous memory locations. The idea is to store multiple items of the same type together. However, the limitation of the array is that the size of the array is predefined and fixed. There are multiple ways to solve this problem. In this article, the diff
5 min read
LinkedList set() Method in Java
The Java.util.LinkedList.set() method is used to replace any particular element in the linked list created using the LinkedList class with another element. This can be done by specifying the position of the element to be replaced and the new element in the parameter of the set() method. Syntax: Link
3 min read
LinkedList add() Method in Java
In Java, the add() method of the LinkedList class is used to add an element to the list. By default, it adds the element to the end of the list, if the index is not specified.Example: Here, we use the add() method to add a single element to the list.Java// Java program to add elements in LinkedList
2 min read
LinkedList removeFirst() Method in Java
In Java, the removeFirst() method of the LinkedList class is used to remove and return the first element of the list.Example 1: Here, we use the removeFirst() method to remove the first element (head) of the LinkedList of Integers.Java// Java Program to demonstrate the // use of removeFirst() in Lin
2 min read
LinkedList remove() Method in Java
In Java, the remove() method of the LinkedList class removes an element from the list, either by specifying its index or by providing its value.Example 1: Here, we use the remove() method to remove element from the LinkedList of Strings. By default the remove() will remove the beginning element(head
3 min read
LinkedList removeLast() Method in Java
In Java, the removeLast() method of LinkedList class is used to remove and return the last element of the list.Example 1: Here, we use the removeLast() method to remove and return the last element (tail) of the LinkedList.Java// java Program to demonstrate the // use of removeLast() in LinkedList im
2 min read
LinkedList get() Method in Java
In Java, the get() method of LinkedList is used to fetch or retrieve an element at a specific index from a LinkedList.Example 1: Here, we use the get() method to retrieve an element at a specified index.Java// Java Program to illustrate get() method import java.util.LinkedList; public class Geeks {
2 min read
LinkedList addAll() Method in Java
In Java, the addAll() method of the LinkedList class is used to add all the elements of one collection to another. This method takes a Collection as an argument and adds all its elements to the end of the list. Example: Here, we use the addAll() method to add all the elements of one collection to an
3 min read
LinkedList push() Method in Java
In Java, the push() method of the LinkedList class is used to push an element at the starting(top) of the stack represented by LinkedList. This is similar to the addFirst() method of LinkedList. This method simply inserts the element at the first position or top of the LinkedList.Syntax of LinkedLis
1 min read