Merge Two Sorted Linked Lists in DSA C - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a new node with given value.

DSA C

struct Node* newNode(int data) {
    struct Node* node = (struct Node*)malloc(sizeof(struct Node));
    node->data = [1];
    node->next = NULL;
    return node;
}

Drag options to blanks, or click blank then click option'

ANULL

Bnode

C0

Ddata

Attempts:

3 left

💡 Hint

Common Mistakes

Assigning NULL or 0 instead of the input data.

Using the node pointer instead of data.

The new node's data field should be set to the input parameter 'data'.

2fill in blank

medium

Complete the code to return the merged list head when one list is empty.

DSA C

struct Node* mergeSortedLists(struct Node* l1, struct Node* l2) {
    if (l1 == NULL) return [1];
    if (l2 == NULL) return l1;
    // merging logic continues...
}

Drag options to blanks, or click blank then click option'

Al1

Bl2

Chead

DNULL

Attempts:

3 left

💡 Hint

Common Mistakes

Returning NULL instead of the other list.

Returning l1 when it is NULL.

If the first list is empty, return the second list as the merged result.

3fill in blank

hard

Fix the error in the comparison to choose the smaller node.

DSA C

if (l1->data [1] l2->data) {
    head = l1;
    head->next = mergeSortedLists(l1->next, l2);
} else {
    head = l2;
    head->next = mergeSortedLists(l1, l2->next);
}

Drag options to blanks, or click blank then click option'

A>

B==

C<

D>=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '>' which picks the larger node first.

Using '==' which only checks equality.

We pick the smaller node, so use '<' to compare data values.

4fill in blank

hard

Fill both blanks to complete the recursive merge function.

DSA C

struct Node* mergeSortedLists(struct Node* l1, struct Node* l2) {
    if (l1 == NULL) return [1];
    if (l2 == NULL) return [2];
    if (l1->data < l2->data) {
        l1->next = mergeSortedLists(l1->next, l2);
        return l1;
    } else {
        l2->next = mergeSortedLists(l1, l2->next);
        return l2;
    }
}

Drag options to blanks, or click blank then click option'

Al2

Bl1

CNULL

Dhead

Attempts:

3 left

💡 Hint

Common Mistakes

Returning NULL instead of the other list.

Swapping the return values.

Return the other list when one is NULL: if l1 is NULL return l2, if l2 is NULL return l1.

5fill in blank

hard

Fill all three blanks to create a function that merges two sorted linked lists.

DSA C

struct Node* mergeSortedLists(struct Node* l1, struct Node* l2) {
    if (l1 == NULL) return [1];
    if (l2 == NULL) return [2];
    if (l1->data [3] l2->data) {
        l1->next = mergeSortedLists(l1->next, l2);
        return l1;
    } else {
        l2->next = mergeSortedLists(l1, l2->next);
        return l2;
    }
}

Drag options to blanks, or click blank then click option'

Al2

Bl1

C<

D>

Attempts:

3 left

💡 Hint

Common Mistakes

Returning NULL instead of the other list.

Using '>' instead of '<' in comparison.

Return the other list if one is NULL, and use '<' to pick the smaller node.

Practice

1. What is the main goal when merging two sorted linked lists?

easy

A. To create a new sorted linked list containing all elements from both lists

B. To reverse both linked lists

C. To find the maximum element in both lists

D. To delete duplicate elements from both lists

Solution

1. Step 1: Understand the merge operation

   Merging two sorted linked lists means combining their elements into one list that remains sorted.

2. Step 2: Identify the goal

   The goal is to create a new sorted linked list that contains all elements from both input lists in order.

3. Final Answer:

   To create a new sorted linked list containing all elements from both lists -> Option A

4. Quick Check:

   Merge = combine sorted lists [OK]

Hint: Merging means combining sorted lists into one sorted list [OK]

Common Mistakes:

• Thinking merge reverses lists
• Confusing merge with finding max element
• Assuming merge removes duplicates

2. Which of the following is the correct way to declare a node structure for a singly linked list in C?

easy

A. struct Node { int data; struct Node* next; };

B. struct Node { int data; Node* next; };

C. struct Node { int data; struct Node next; };

D. struct Node { int data; struct Node* prev; };

Solution

1. Step 1: Check struct syntax for linked list node

   In C, a node contains data and a pointer to the next node. The pointer must be declared as 'struct Node* next;'.

2. Step 2: Identify correct declaration

   struct Node { int data; struct Node* next; }; correctly declares 'struct Node' with 'int data' and 'struct Node* next;'. struct Node { int data; Node* next; }; misses 'struct' keyword before Node*, struct Node { int data; struct Node next; }; incorrectly uses 'struct Node next;' (not a pointer), and struct Node { int data; struct Node* prev; }; uses 'prev' which is for doubly linked lists.

3. Final Answer:

   struct Node { int data; struct Node* next; }; -> Option A

4. Quick Check:

   Node struct needs pointer to next node [OK]

Hint: Node pointer must use 'struct Node*' in C [OK]

Common Mistakes:

• Omitting 'struct' keyword before Node*
• Using node instead of pointer for next
• Confusing singly linked list with doubly linked list

3. What will be the output linked list after merging these two sorted linked lists?

List1: 1 -> 3 -> 5 -> NULL
List2: 2 -> 4 -> 6 -> NULL

Assuming the merge function merges correctly.

medium

A. 1 -> 3 -> 5 -> 2 -> 4 -> 6 -> NULL

B. 2 -> 4 -> 6 -> 1 -> 3 -> 5 -> NULL

C. 1 -> 2 -> 3 -> 4 -> 5 -> 6 -> NULL

D. 6 -> 5 -> 4 -> 3 -> 2 -> 1 -> NULL

Solution

1. Step 1: Compare elements from both lists in order

   Start with heads: 1 (List1) and 2 (List2). 1 < 2, so 1 goes first. Then compare 3 and 2, 2 < 3, so 2 goes next. Continue this way to merge in sorted order.

2. Step 2: Build merged list step-by-step

   Merged list: 1 -> 2 -> 3 -> 4 -> 5 -> 6 -> NULL.

3. Final Answer:

   1 -> 2 -> 3 -> 4 -> 5 -> 6 -> NULL -> Option C

4. Quick Check:

   Merge keeps sorted order [OK]

Hint: Pick smaller node each step to merge sorted lists [OK]

Common Mistakes:

• Appending one list after the other without sorting
• Reversing the order accidentally
• Skipping nodes during merge

4. Consider this buggy merge function snippet in C:

struct Node* merge(struct Node* l1, struct Node* l2) {
    if (!l1) return l2;
    if (!l2) return l1;
    if (l1->data < l2->data) {
        l1->next = merge(l1->next, l2);
        return l1;
    } else {
        l2->next = merge(l1, l2->next);
        return l2;
    }
}

What is the main issue if this function is called with two sorted lists?

medium

A. It reverses the merged list

B. It merges lists correctly without issues

C. It loses nodes when one list is empty

D. It causes infinite recursion when lists have equal elements

Solution

1. Step 1: Analyze base cases

   If one list is empty (!l1 or !l2), it correctly returns the other list.

2. Step 2: Analyze recursive case

   If l1->data < l2->data, it sets l1->next to the merge of the rest and returns l1. Otherwise, sets l2->next to merge(l1, l2->next) and returns l2. However, when l1->data == l2->data, the else branch is taken repeatedly without advancing l1, causing infinite recursion.

3. Final Answer:

   It causes infinite recursion when lists have equal elements -> Option D

4. Quick Check:

   Equal elements cause infinite recursion due to no advancement in l1 [OK]

Hint: Recursive merge must handle equal elements to avoid infinite recursion [OK]

Common Mistakes:

• Thinking equal elements cause infinite recursion
• Believing it loses nodes on empty lists
• Assuming it reverses the list

5. You have two sorted linked lists:
List1: 1 -> 4 -> 7 -> NULL
List2: 2 -> 4 -> 6 -> 8 -> NULL

Write the merged sorted linked list after merging them and removing duplicate values.

hard

A. 1 -> 4 -> 7 -> 2 -> 4 -> 6 -> 8 -> NULL

B. 1 -> 2 -> 4 -> 6 -> 7 -> 8 -> NULL

C. 1 -> 2 -> 4 -> 4 -> 6 -> 7 -> 8 -> NULL

D. 1 -> 2 -> 6 -> 7 -> 8 -> NULL

Solution

1. Step 1: Merge lists in sorted order

   Merge elements: 1, 2, 4, 4, 6, 7, 8.

2. Step 2: Remove duplicate values

   Remove one of the duplicate 4's to get: 1 -> 2 -> 4 -> 6 -> 7 -> 8 -> NULL.

3. Final Answer:

   1 -> 2 -> 4 -> 6 -> 7 -> 8 -> NULL -> Option B

4. Quick Check:

   Merge + remove duplicates = sorted unique list [OK]

Hint: Merge then skip duplicates for unique sorted list [OK]

Common Mistakes:

• Leaving duplicates in merged list
• Appending lists without sorting
• Removing unique elements by mistake