Traversal Forward and Backward in DSA Python - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Forward and Backward Traversal in Doubly Linked List

What is the printed output after running the following code that traverses a doubly linked list forward and then backward?

DSA Python

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

class DoublyLinkedList:
    def __init__(self):
        self.head = None
        self.tail = None

    def append(self, val):
        new_node = Node(val)
        if not self.head:
            self.head = new_node
            self.tail = new_node
        else:
            self.tail.next = new_node
            new_node.prev = self.tail
            self.tail = new_node

    def traverse_forward(self):
        current = self.head
        result = []
        while current:
            result.append(str(current.val))
            current = current.next
        return ' -> '.join(result) + ' -> null'

    def traverse_backward(self):
        current = self.tail
        result = []
        while current:
            result.append(str(current.val))
            current = current.prev
        return ' -> '.join(result) + ' -> null'

# Create list and append values
dll = DoublyLinkedList()
dll.append(10)
dll.append(20)
dll.append(30)

# Print forward and backward traversal
print(dll.traverse_forward())
print(dll.traverse_backward())

A "10 -> 20 -> 30 -> null\n10 -> 20 -> 30 -> null"

B "10 -> 20 -> 30 -> null\n30 -> 20 -> 10 -> null"

C "30 -> 20 -> 10 -> null\n10 -> 20 -> 30 -> null"

D "30 -> 20 -> 10 -> null\n30 -> 20 -> 10 -> null"

💡 Hint

Think about how the doubly linked list stores references to both next and previous nodes.

The forward traversal prints nodes from head to tail: 10 -> 20 -> 30 -> null. The backward traversal prints nodes from tail to head: 30 -> 20 -> 10 -> null.

🧠 Conceptual

intermediate

Understanding Traversal Direction in Doubly Linked Lists

In a doubly linked list, which pointer is used to move backward during traversal?

A The 'next' pointer

B The 'tail' pointer

C The 'head' pointer

D The 'prev' pointer

💡 Hint

Backward means going to the previous node.

The 'prev' pointer in each node points to the previous node, allowing backward traversal.

🔧 Debug

advanced

Identify the Error in Backward Traversal Code

What is wrong with this backward traversal code snippet on a doubly linked list?

DSA Python

def traverse_backward(head):
    current = head
    result = []
    while current:
        result.append(str(current.val))
        current = current.prev
    return ' -> '.join(result) + ' -> null'

# Assume doubly linked list with head node exists

A "Logical error: starts from 'head' instead of 'tail', only visits the head node"

B Infinite loop because current never becomes None

C No error, returns correct backward traversal

D TypeError because 'prev' is not a valid attribute

💡 Hint

Think about the starting point for backward traversal.

Backward traversal must start from the tail node, not the head. Starting at head only visits the head node since head.prev is None immediately after the first iteration. No runtime error occurs.

❓ Predict Output

advanced

Output After Mixed Forward and Backward Traversal

What is the output of the following code that traverses a doubly linked list forward, then backward, then forward again?

DSA Python

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

class DoublyLinkedList:
    def __init__(self):
        self.head = None
        self.tail = None

    def append(self, val):
        new_node = Node(val)
        if not self.head:
            self.head = new_node
            self.tail = new_node
        else:
            self.tail.next = new_node
            new_node.prev = self.tail
            self.tail = new_node

    def traverse_forward(self):
        current = self.head
        result = []
        while current:
            result.append(str(current.val))
            current = current.next
        return ' -> '.join(result) + ' -> null'

    def traverse_backward(self):
        current = self.tail
        result = []
        while current:
            result.append(str(current.val))
            current = current.prev
        return ' -> '.join(result) + ' -> null'

# Create list and append values
dll = DoublyLinkedList()
dll.append(1)
dll.append(2)
dll.append(3)

print(dll.traverse_forward())
print(dll.traverse_backward())
print(dll.traverse_forward())

A "1 -> 2 -> 3 -> null\n1 -> 2 -> 3 -> null\n3 -> 2 -> 1 -> null"

B "3 -> 2 -> 1 -> null\n1 -> 2 -> 3 -> null\n3 -> 2 -> 1 -> null"

C "1 -> 2 -> 3 -> null\n3 -> 2 -> 1 -> null\n1 -> 2 -> 3 -> null"

D "3 -> 2 -> 1 -> null\n3 -> 2 -> 1 -> null\n1 -> 2 -> 3 -> null"

💡 Hint

The list does not change between traversals.

The list is traversed forward (1->2->3), then backward (3->2->1), then forward again (1->2->3).

🚀 Application

expert

Number of Nodes Visited in Mixed Traversal

Given a doubly linked list with 5 nodes, if you traverse forward 3 nodes starting from head, then backward 2 nodes, how many unique nodes have you visited in total?

A 3 unique nodes

B 4 unique nodes

C 5 unique nodes

D 2 unique nodes

💡 Hint

Count nodes visited without repeating.

Assume nodes labeled 1-2-3-4-5. Forward traversal visits nodes 1, 2, 3 (ending at 3). Backward 2 nodes from there visits 2, then 1. Unique nodes: 1, 2, 3.