Push Using Linked List Node in DSA C - Build from Scratch

Push Using Linked List Node

📖 Scenario: You are building a simple stack using a linked list in C. A stack is like a pile of plates where you add new plates on top. Here, each plate is a node in the linked list.

🎯 Goal: Build a linked list stack and add a new node to the top using the push operation.

📋 What You'll Learn

Create a struct called Node with an int data and a Node* next pointer.

Create a pointer called top to represent the top of the stack and initialize it to NULL.

Create a new node with data = 10 and push it onto the stack by updating top.

Print the data of the top node after the push operation.

💡 Why This Matters

🌍 Real World

Stacks are used in many programs like undo features, expression evaluation, and backtracking algorithms.

💼 Career

Understanding linked list operations like push is essential for software developers working with low-level data structures and memory management.

1

Create the Node struct and initialize the top pointer

Write the struct definition for Node with an int data and a Node* next. Then create a pointer called top and set it to NULL.

DSA C

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

struct Node* top = NULL;
// Your code here

Hint

Use struct Node to define the node. Initialize top as a pointer to Node and set it to NULL.

2

Create a new node with data 10

Create a new Node pointer called newNode. Allocate memory for it using malloc. Set its data to 10 and its next to NULL.

DSA C

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

struct Node* top = NULL;

// Create newNode and set data to 10
// Your code here

Hint

Use malloc(sizeof(struct Node)) to allocate memory. Use arrow -> to set data and next.

3

Push the new node onto the stack

Set the next of newNode to the current top. Then update top to point to newNode.

DSA C

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

struct Node* top = NULL;

struct Node* newNode = (struct Node*)malloc(sizeof(struct Node));
newNode->data = 10;
newNode->next = NULL;

// Push newNode onto stack
// Your code here

Hint

To push, link newNode->next to top and then update top to newNode.

4

Print the data of the top node

Use printf to print the data of the top node in this format: Top data: 10\n.

DSA C

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

struct Node* top = NULL;

struct Node* newNode = (struct Node*)malloc(sizeof(struct Node));
newNode->data = 10;
newNode->next = NULL;

newNode->next = top;
top = newNode;

// Print top node data
// Your code here

Hint

Use printf("Top data: %d\n", top->data); to print the data of the top node.

Practice

1. What does the push operation do in a linked list when using nodes?

easy

A. Searches for a node with a specific value

B. Adds a new node at the beginning of the list

C. Removes the last node from the list

D. Deletes the entire linked list

Solution

1. Step 1: Understand the push operation

   The push operation adds a new element to the linked list.

2. Step 2: Identify where the new node is added

   In a linked list, push adds the new node at the start (head) of the list.

3. Final Answer:

   Adds a new node at the beginning of the list -> Option B

4. Quick Check:

   Push = Add at head [OK]

Hint: Push always adds at the start of the linked list [OK]

Common Mistakes:

• Thinking push adds at the end
• Confusing push with pop
• Assuming push deletes nodes

2. Which of the following is the correct syntax to create a new node and push it onto the linked list in C?

easy

A. newNode = malloc(sizeof(Node)); newNode->prev = head; head = newNode;

B. newNode = malloc(Node); newNode->next = head; head = newNode;

C. newNode = malloc(sizeof(Node)); newNode->next = head; head = newNode;

D. newNode = malloc(sizeof(Node)); head = newNode->next; newNode = head;

Solution

1. Step 1: Allocate memory correctly for new node

   Use malloc(sizeof(Node)) to allocate memory for the new node.

2. Step 2: Link new node to current head and update head

   Set newNode->next to current head, then update head to newNode.

3. Final Answer:

   newNode = malloc(sizeof(Node)); newNode->next = head; head = newNode; -> Option C

4. Quick Check:

   Correct malloc and linking = newNode = malloc(sizeof(Node)); newNode->next = head; head = newNode; [OK]

Hint: Use sizeof(Node) with malloc and link newNode->next to head [OK]

Common Mistakes:

• Forgetting sizeof in malloc
• Assigning head before setting newNode->next
• Using prev pointer in singly linked list

3. What will be the linked list after executing the following code?

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

Node* head = NULL;

void push(int val) {
  Node* newNode = malloc(sizeof(Node));
  newNode->data = val;
  newNode->next = head;
  head = newNode;
}

push(10);
push(20);
push(30);

medium

A. 30 -> 20 -> 10 -> null

B. 10 -> 20 -> 30 -> null

C. 20 -> 30 -> 10 -> null

D. 10 -> 30 -> 20 -> null

Solution

1. Step 1: Trace each push call

   push(10) adds 10 at head: 10 -> null; push(20) adds 20 at head: 20 -> 10 -> null; push(30) adds 30 at head: 30 -> 20 -> 10 -> null.

2. Step 2: Confirm final linked list order

   The last pushed value is at the head, so list is 30 -> 20 -> 10 -> null.

3. Final Answer:

   30 -> 20 -> 10 -> null -> Option A

4. Quick Check:

   Last pushed is head = 30 -> 20 -> 10 [OK]

Hint: Last pushed node is always at the head [OK]

Common Mistakes:

• Assuming push adds at tail
• Reading list in wrong order
• Confusing push with append

4. Identify the error in this push function and select the correct fix:

void push(int val) {
  Node* newNode;
  newNode->data = val;
  newNode->next = head;
  head = newNode;
}

medium

A. head should be assigned before newNode->next

B. newNode->data should be assigned after head update

C. newNode->next should be NULL instead of head

D. newNode is not allocated memory; fix by malloc(sizeof(Node))

Solution

1. Step 1: Check newNode initialization

   newNode is declared but not allocated memory with malloc, so it points to garbage.

2. Step 2: Fix by allocating memory

   Use newNode = malloc(sizeof(Node)) before accessing newNode->data or newNode->next.

3. Final Answer:

   newNode is not allocated memory; fix by malloc(sizeof(Node)) -> Option D

4. Quick Check:

   Always malloc before using newNode [OK]

Hint: Always allocate memory before using new node [OK]

Common Mistakes:

• Using uninitialized pointer
• Assigning data before allocation
• Changing head before linking newNode

5. You want to push multiple values 5, 15, 25 onto an empty linked list using the push function below. After pushing, you want to print the list from head to tail. Which code snippet correctly performs this and prints the list?

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

Node* head = NULL;

void push(int val) {
  Node* newNode = malloc(sizeof(Node));
  newNode->data = val;
  newNode->next = head;
  head = newNode;
}

// Your code here to push and print

hard

A. push(5); push(15); push(25); Node* temp = head; while(temp) { printf("%d -> ", temp->data); temp = temp->next; } printf("null\n");

B. push(5); push(15); push(25); Node* temp = head; while(temp->next) { printf("%d -> ", temp->data); temp = temp->next; } printf("null\n");

C. push(25); push(15); push(5); Node* temp = head; while(temp) { printf("%d -> ", temp->data); temp = temp->next; } printf("null\n");

D. push(5); push(15); push(25); Node* temp = head; while(temp) { printf("%d ", temp->data); temp = temp->next; }

Solution

1. Step 1: Push values in correct order

   Push 5, then 15, then 25. Each push adds at head, so list is 25 -> 15 -> 5 -> null.

2. Step 2: Print all nodes correctly

   Use a loop while(temp) to print each node's data followed by '->', then print 'null' after loop.

3. Final Answer:

   push(5); push(15); push(25); Node* temp = head; while(temp) { printf("%d -> ", temp->data); temp = temp->next; } printf("null\n"); -> Option A

4. Quick Check:

   Push order + full print loop = push(5); push(15); push(25); Node* temp = head; while(temp) { printf("%d -> ", temp->data); temp = temp->next; } printf("null\n"); [OK]

Hint: Push in order, print with while(temp) loop ending with null [OK]

Common Mistakes:

• Pushing in reverse order
• Using while(temp->next) and missing last node
• Not printing 'null' at end