Get Length of Linked List in DSA C - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a linked list?

A linked list is a chain of nodes where each node holds data and a link (pointer) to the next node. It is like a treasure hunt where each clue leads to the next.

beginner

How do you find the length of a linked list?

Start at the first node and move to the next node one by one, counting each node until you reach the end (where the next pointer is NULL). The count is the length.

beginner

What does the 'NULL' pointer represent in a linked list?

NULL means there is no next node. It marks the end of the linked list, like a 'The End' sign in a story.

beginner

Why do we use a loop to get the length of a linked list?

Because we need to visit each node one by one until the end. A loop helps us repeat this step easily without missing any node.

intermediate

What is the time complexity of getting the length of a linked list?

It is O(n), where n is the number of nodes. We must check each node once to count them all.

What does the length of a linked list represent?

A Pointer to the last node

B Value of the first node

C Number of nodes in the list

D Memory size of the list

The length is the count of all nodes in the linked list.

What condition tells us we reached the end of a linked list?

A Node value is negative

B Data is zero

C Previous pointer is NULL

D Next pointer is NULL

The next pointer being NULL means no more nodes follow.

Which loop is commonly used to traverse a linked list?

A for loop

B while loop

C do-while loop

D infinite loop

A while loop is used to move through nodes until NULL is found.

If a linked list has 5 nodes, what will the length function return?

A 5

B 4

C 6

D 0

The length is the total number of nodes, which is 5.

What is the initial value of the count variable when finding length?

A 0

B 1

C NULL

D Undefined

Count starts at 0 before counting nodes.

Explain step-by-step how to find the length of a linked list.

Think of walking through a chain counting each link.

Why is it important to check for NULL when traversing a linked list?

NULL is like a stop sign on the path.

Practice

1. What does the function to get the length of a linked list typically do?

easy

A. Reverses the linked list

B. Deletes all nodes in the linked list

C. Finds the maximum value stored in the linked list

D. Counts the number of nodes by traversing from head to end

Solution

1. Step 1: Understand the linked list traversal

   The function starts at the head node and moves through each node until it reaches the end (null).

2. Step 2: Count each node visited

   Each time the function moves to the next node, it increases a counter by one.

3. Final Answer:

   Counts the number of nodes by traversing from head to end -> Option D

4. Quick Check:

   Length = number of nodes visited [OK]

Hint: Count nodes while moving from head to null [OK]

Common Mistakes:

• Confusing length with maximum value
• Trying to reverse the list instead
• Not traversing all nodes

2. Which of the following is the correct C syntax to declare a pointer to the head of a linked list node named head?

easy

A. struct Node* head;

B. struct Node &head;

C. Node head*;

D. struct Node head;

Solution

1. Step 1: Recall pointer declaration in C

   To declare a pointer to a struct Node, use struct Node* variableName;.

2. Step 2: Match with options

   struct Node* head; correctly declares head as a pointer to struct Node.

3. Final Answer:

   struct Node* head; -> Option A

4. Quick Check:

   Pointer syntax uses * after type [OK]

Hint: Pointer declaration uses * after type name [OK]

Common Mistakes:

• Forgetting the * for pointer
• Using & which is reference in C++ not C
• Incorrect order of * and variable name

3. What will be the output of the following C code snippet?

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

int getLength(struct Node* head) {
    int count = 0;
    while (head != NULL) {
        count++;
        head = head->next;
    }
    return count;
}

int main() {
    struct Node n3 = {30, NULL};
    struct Node n2 = {20, &n3};
    struct Node n1 = {10, &n2};
    printf("%d", getLength(&n1));
    return 0;
}

medium

A. 3

B. 2

C. 1

D. 0

Solution

1. Step 1: Analyze linked list nodes

   There are three nodes: n1 -> n2 -> n3 -> NULL.

2. Step 2: Count nodes in getLength function

   The function counts each node until it reaches NULL, so count = 3.

3. Final Answer:

   3 -> Option A

4. Quick Check:

   3 nodes counted from head to end [OK]

Hint: Count nodes until next is NULL [OK]

Common Mistakes:

• Counting only head node
• Off-by-one error in counting
• Confusing pointer addresses with count

4. Find the error in this C function to get the length of a linked list:

int getLength(struct Node* head) {
    int count = 0;
    while (head->next != NULL) {
        count++;
        head = head->next;
    }
    return count;
}

medium

A. It causes a segmentation fault immediately

B. It misses counting the last node

C. It counts nodes twice

D. It returns the data of the last node

Solution

1. Step 1: Understand the loop condition

   The loop runs while head->next != NULL, so it stops before the last node.

2. Step 2: Check counting logic

   Because it stops early, the last node is not counted, so count is one less than actual length.

3. Final Answer:

   It misses counting the last node -> Option B

4. Quick Check:

   Loop condition excludes last node [OK]

Hint: Use while(head != NULL) to count all nodes [OK]

Common Mistakes:

• Using head->next in loop condition
• Not handling empty list (NULL head)
• Incrementing count after loop ends

5. You have a linked list where some nodes may have next pointers forming a cycle (loop). How would you modify the length function to avoid infinite counting?

hard

A. Stop counting after 10 nodes to avoid infinite loop

B. Count nodes normally without any change

C. Use two pointers moving at different speeds to detect a cycle before counting

D. Set all next pointers to NULL before counting

Solution

1. Step 1: Detect cycle using two pointers

   Use Floyd's cycle detection: one pointer moves one step, another moves two steps to find a loop.

2. Step 2: If cycle detected, handle counting carefully

   Once a cycle is found, count nodes up to the cycle start to avoid infinite counting.

3. Final Answer:

   Use two pointers moving at different speeds to detect a cycle before counting -> Option C

4. Quick Check:

   Cycle detection prevents infinite loops [OK]

Hint: Detect cycle first with slow and fast pointers [OK]

Common Mistakes:

• Ignoring cycle causes infinite loop
• Stopping count arbitrarily without detection
• Modifying list structure destructively