Reverse a LinkedList in Java

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.
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// 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;
    }
}

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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
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// 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;
    }
}

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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)”

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// 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);
    }
}

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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.

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// Java program for reversing a inkedlist 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);
        }
    }
}

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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.



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