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

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Forward Traversal in Doubly Linked List

What is the printed output of the following C code that traverses a doubly linked list forward?

DSA C

#include <stdio.h>
#include <stdlib.h>

typedef struct Node {
    int data;
    struct Node* prev;
    struct Node* next;
} Node;

void printForward(Node* head) {
    Node* current = head;
    while (current != NULL) {
        printf("%d -> ", current->data);
        current = current->next;
    }
    printf("NULL\n");
}

int main() {
    Node* n1 = malloc(sizeof(Node));
    Node* n2 = malloc(sizeof(Node));
    Node* n3 = malloc(sizeof(Node));

    n1->data = 10; n1->prev = NULL; n1->next = n2;
    n2->data = 20; n2->prev = n1; n2->next = n3;
    n3->data = 30; n3->prev = n2; n3->next = NULL;

    printForward(n1);
    return 0;
}

A 10 -> 20 -> 30 -> NULL

B 30 -> 20 -> 10 -> NULL

C 10 20 30 NULL

D NULL -> 10 -> 20 -> 30

💡 Hint

Follow the next pointers starting from the head node.

The function printForward starts at the head and moves through the next pointers, printing each node's data followed by '->'. The list is 10, 20, 30 in order, ending with NULL.

❓ Predict Output

intermediate

Output of Backward Traversal in Doubly Linked List

What is the printed output of the following C code that traverses a doubly linked list backward?

DSA C

#include <stdio.h>
#include <stdlib.h>

typedef struct Node {
    int data;
    struct Node* prev;
    struct Node* next;
} Node;

void printBackward(Node* tail) {
    Node* current = tail;
    while (current != NULL) {
        printf("%d -> ", current->data);
        current = current->prev;
    }
    printf("NULL\n");
}

int main() {
    Node* n1 = malloc(sizeof(Node));
    Node* n2 = malloc(sizeof(Node));
    Node* n3 = malloc(sizeof(Node));

    n1->data = 10; n1->prev = NULL; n1->next = n2;
    n2->data = 20; n2->prev = n1; n2->next = n3;
    n3->data = 30; n3->prev = n2; n3->next = NULL;

    printBackward(n3);
    return 0;
}

A 30 -> 20 -> 10 -> NULL

B 10 -> 20 -> 30 -> NULL

C NULL -> 30 -> 20 -> 10

D 30 20 10 NULL

💡 Hint

Follow the prev pointers starting from the tail node.

The function printBackward starts at the tail and moves through the prev pointers, printing each node's data followed by '->'. The list backward is 30, 20, 10 ending with NULL.

🧠 Conceptual

advanced

Understanding Traversal Pointers in Doubly Linked List

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

A head pointer

B next pointer

C prev pointer

D tail pointer

💡 Hint

Think about which pointer links to the previous node.

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

🔧 Debug

advanced

Identify the Bug in Backward Traversal Code

What error will occur when running this backward traversal code snippet?

DSA C

#include <stdio.h>
#include <stdlib.h>

typedef struct Node {
    int data;
    struct Node* prev;
    struct Node* next;
} Node;

void printBackward(Node* tail) {
    Node* current = tail;
    while (current != NULL) {
        printf("%d -> ", current->next->data);
        current = current->prev;
    }
    printf("NULL\n");
}

A Compilation error due to missing semicolon

B Segmentation fault (accessing NULL pointer)

C No error, prints backward list correctly

D Infinite loop

💡 Hint

Check what happens when current->next is NULL.

Accessing current->next->data when current->next is NULL causes a segmentation fault.

🚀 Application

expert

Result of Mixed Forward and Backward Traversal

Given a doubly linked list with nodes 1 <-> 2 <-> 3 <-> 4, what is the output after this code runs? 1. Traverse forward from head and print nodes. 2. Then traverse backward from tail and print nodes. Assume print format: "data -> " and ends with "NULL".

DSA C

#include <stdio.h>
#include <stdlib.h>

typedef struct Node {
    int data;
    struct Node* prev;
    struct Node* next;
} Node;

void printForward(Node* head) {
    Node* current = head;
    while (current != NULL) {
        printf("%d -> ", current->data);
        current = current->next;
    }
    printf("NULL\n");
}

void printBackward(Node* tail) {
    Node* current = tail;
    while (current != NULL) {
        printf("%d -> ", current->data);
        current = current->prev;
    }
    printf("NULL\n");
}

int main() {
    Node* n1 = malloc(sizeof(Node));
    Node* n2 = malloc(sizeof(Node));
    Node* n3 = malloc(sizeof(Node));
    Node* n4 = malloc(sizeof(Node));

    n1->data = 1; n1->prev = NULL; n1->next = n2;
    n2->data = 2; n2->prev = n1; n2->next = n3;
    n3->data = 3; n3->prev = n2; n3->next = n4;
    n4->data = 4; n4->prev = n3; n4->next = NULL;

    printForward(n1);
    printBackward(n4);
    return 0;
}

A

NULL -> 1 -> 2 -> 3 -> 4
NULL -> 4 -> 3 -> 2 -> 1

B

4 -> 3 -> 2 -> 1 -> NULL
1 -> 2 -> 3 -> 4 -> NULL

C

1 2 3 4 NULL
4 3 2 1 NULL

D

1 -> 2 -> 3 -> 4 -> NULL
4 -> 3 -> 2 -> 1 -> NULL

💡 Hint

First print forward from head, then backward from tail.

The first printForward prints nodes 1 to 4 in order, the second printBackward prints nodes 4 to 1 in reverse order, both ending with NULL.

Practice

1. What does forward traversal in a linked list mean?

easy

A. Visiting nodes from the first node to the last node

B. Visiting nodes from the last node to the first node

C. Skipping every alternate node while visiting

D. Visiting nodes randomly

Solution

1. Step 1: Understand forward traversal

   Forward traversal means starting at the first node and moving to the next node repeatedly until the end.

2. Step 2: Compare with options

   Only Visiting nodes from the first node to the last node describes visiting nodes from first to last, which matches forward traversal.

3. Final Answer:

   Visiting nodes from the first node to the last node -> Option A

4. Quick Check:

   Forward traversal = first to last [OK]

Hint: Forward means start to end in order [OK]

Common Mistakes:

• Confusing forward with backward traversal
• Thinking traversal is random
• Assuming skipping nodes is traversal

2. Which of the following is the correct syntax to traverse a singly linked list forward in C?

easy

A. for(Node* curr = head; curr != NULL; curr = curr->prev) { /* process curr */ }

B. for(Node* curr = head; curr != NULL; curr = curr->next) { /* process curr */ }

C. while(head != NULL) { head = head->prev; }

D. for(Node* curr = tail; curr != NULL; curr = curr->prev) { /* process curr */ }

Solution

1. Step 1: Identify forward traversal syntax

   Forward traversal in singly linked list uses next pointer starting from head.

2. Step 2: Check options for correct pointer usage

   for(Node* curr = head; curr != NULL; curr = curr->next) { /* process curr */ } uses curr = curr->next starting from head, which is correct.

3. Final Answer:

   for(Node* curr = head; curr != NULL; curr = curr->next) { /* process curr */ } -> Option B

4. Quick Check:

   Use next pointer from head for forward traversal [OK]

Hint: Use next pointer from head for forward traversal [OK]

Common Mistakes:

• Using prev pointer in singly linked list
• Starting from tail in singly linked list
• Using incorrect loop conditions

3. What will be the output of the following code snippet that traverses a doubly linked list backward?

typedef struct Node {
  int data;
  struct Node* next;
  struct Node* prev;
} Node;

void printBackward(Node* tail) {
  Node* curr = tail;
  while(curr != NULL) {
    printf("%d ", curr->data);
    curr = curr->prev;
  }
}

// List: 1 <-> 2 <-> 3, tail points to node with data 3
printBackward(tail);

medium

A. 1 2 3

B. 3 1 2

C. 3 2 1

D. 2 3 1

Solution

1. Step 1: Understand backward traversal in doubly linked list

   Starting from tail (3), moving prev pointer visits nodes 3, 2, then 1.

2. Step 2: Trace the output

   Prints 3, then 2, then 1 with spaces.

3. Final Answer:

   3 2 1 -> Option C

4. Quick Check:

   Backward traversal prints tail to head [OK]

Hint: Backward traversal prints from tail to head [OK]

Common Mistakes:

• Confusing order of printing
• Using next pointer instead of prev
• Starting from head instead of tail

4. Identify the error in the following code that attempts to traverse a doubly linked list backward:

void traverseBackward(Node* tail) {
  Node* curr = tail;
  while(curr->next != NULL) {
    printf("%d ", curr->data);
    curr = curr->prev;
  }
}

medium

A. The printf statement is missing a newline character

B. The pointer curr should start from head, not tail

C. The code should use curr = curr->next instead of curr = curr->prev

D. The loop condition should check curr != NULL, not curr->next != NULL

Solution

1. Step 1: Analyze loop condition for backward traversal

   Backward traversal moves using prev pointer until curr becomes NULL.

2. Step 2: Identify incorrect condition

   Checking curr->next != NULL is wrong; it should be curr != NULL to avoid null pointer access.

3. Final Answer:

   The loop condition should check curr != NULL, not curr->next != NULL -> Option D

4. Quick Check:

   Loop until curr is NULL in backward traversal [OK]

Hint: Check curr != NULL in loops, not curr->next for backward traversal [OK]

Common Mistakes:

• Using next pointer condition in backward traversal
• Starting from head instead of tail
• Ignoring null pointer checks

5. Given a doubly linked list with nodes containing integers, write a function to print the list forward and then backward without using extra space. Which approach correctly achieves this?

hard

A. Traverse from head to tail printing data, then traverse from tail to head printing data

B. Traverse from head to tail printing data twice

C. Traverse from tail to head printing data, then traverse from head to tail printing data

D. Traverse from head to tail printing data, then reverse the list and print forward again

Solution

1. Step 1: Understand traversal without extra space

   Use existing next and prev pointers to traverse forward and backward.

2. Step 2: Identify correct approach

   Traverse from head to tail printing data, then traverse from tail to head printing data correctly traverses forward from head to tail, then backward from tail to head printing data without modifying list.

3. Final Answer:

   Traverse from head to tail printing data, then traverse from tail to head printing data -> Option A

4. Quick Check:

   Use head and tail pointers for forward and backward traversal [OK]

Hint: Print forward from head, backward from tail without changing list [OK]

Common Mistakes:

• Reversing list unnecessarily
• Printing forward twice
• Using extra space like arrays or stacks