Python Program to Find Last 3rd element in Singly Linked List

In this tutorial, we are going to learn the writing python program to Find 3rd element of Linked List from last in Singly Linked List.

Python Program to find last 3rd element in Singly linked list

class Node:
    def __init__(self, data):
        self.data = data
        self.next = None


class SinglyLinkedList:

    def __init__(self):
        self.head = None

    def push(self, new_data):
        """Add element at the beginning"""
        new_node = Node(new_data)
        new_node.next = self.head
        self.head = new_node

    def pushLast(self, new_data):
        """Add element at the end"""
        new_node = Node(new_data)
        if self.head is None:
            self.head = new_node
            return
        temp = self.head
        while temp.next:
            temp = temp.next
        temp.next = new_node

    def findThirdFromEnd(self):
        """Find 3rd element from end using single pass (two-pointer approach)"""
        first = self.head
        second = self.head

        # Move second pointer 3 steps ahead
        for i in range(3):
            if second is None:
                print("Linked list has less than 3 elements.")
                return
            second = second.next

        # Move both pointers until second reaches the end
        while second:
            first = first.next
            second = second.next

        print("3rd element from the end is:", first.data)

    def __str__(self):
        """Return string representation of the list"""
        result = []
        temp = self.head
        while temp:
            result.append(str(temp.data))
            temp = temp.next
        return " -> ".join(result) if result else "[Empty List]"


# Main program
s = SinglyLinkedList()

print("Linked List Menu:")
print("1: Add element at the beginning")
print("2: Add element at the end")
print("3: Find 3rd element from the end")
print("4: Display the linked list")
print("0: Exit")

while True:
    try:
        choice = int(input("\nEnter your choice: "))
    except ValueError:
        print("[INVALID INPUT]: Please enter a valid number.")
        continue

    if choice == 0:
        print("Exiting program.")
        break
    elif choice == 1:
        data = input("Enter element to add at beginning: ")
        s.push(data)
    elif choice == 2:
        data = input("Enter element to add at end: ")
        s.pushLast(data)
    elif choice == 3:
        s.findThirdFromEnd()
    elif choice == 4:
        print("Linked list contents:")
        print(s)
    else:
        print("[INVALID INPUT]: Please enter a valid menu option.")

Output

Linked List Menu:
1: Add element at the beginning
2: Add element at the end
3: Find 3rd element from the end
4: Display the linked list
0: Exit

Enter your choice: 1
Enter element to add at beginning: 3

Enter your choice: 1
Enter element to add at beginning: 5

Enter your choice: 1
Enter element to add at beginning: 8

Enter your choice: 2
Enter element to add at end: 9

Enter your choice: 4
Linked list contents:
8 -> 5 -> 3 -> 9

Enter your choice: 3
3rd element from the end is: 5

Enter your choice: 0
Exiting program.