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. Each new item is added to the top of the stack by pushing a new node.

🎯 Goal: Create a linked list stack and implement the push operation by adding a new node at the front.

📋 What You'll Learn

Create a Node struct with int data and Node* next

Create a pointer head to track the top of the stack

Create a function push that takes an int value and adds a new node at the front

Print the linked list after pushing nodes to show the stack state

💡 Why This Matters

🌍 Real World

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

💼 Career

Understanding linked list operations like push is essential for many software engineering roles involving data structures.

1

Create Node struct and head pointer

Create a struct Node with int data and Node* next. Then create a Node* head pointer and set it to nullptr.

DSA C++

// Define Node struct and head pointer
// Your code here

Hint

Think of Node as a box holding a number and a pointer to the next box.

2

Create push function to add new node

Create a function void push(int value) that creates a new Node, sets its data to value, sets its next to head, and updates head to this new node.

DSA C++

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

Node* head = nullptr;

// Define push function to add new node at front
// Your code here

Hint

Remember to link the new node to the current head before moving the head pointer.

3

Push values 10 and 20 onto the stack

Call push(10) and then push(20) to add two nodes to the stack.

DSA C++

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

Node* head = nullptr;

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

// Push 10 and 20 onto the stack
// Your code here

Hint

Call the push function twice with the exact values 10 and 20.

4

Print the stack after pushing nodes

Write a while loop to print the stack starting from head. Print each node's data followed by ->. End with null.

DSA C++

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

Node* head = nullptr;

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

int main() {
    push(10);
    push(20);
    // Print the stack
    // Your code here
    return 0;
}

Hint

Use a pointer to walk through the list from head to the end, printing each node's data.

Practice

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

easy

A. Deletes the entire linked list

B. Removes the last node from the list

C. Searches for a node with a specific value

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

Solution

1. Step 1: Understand push operation in linked list

   The push operation inserts a new node at the start (head) of the linked list.

2. Step 2: Effect of push on list structure

   The new node becomes the head, and its next pointer points to the previous head node.

3. Final Answer:

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

4. Quick Check:

   Push means insert at head = D [OK]

Hint: Push always adds node at the start (head) [OK]

Common Mistakes:

• Confusing push with pop (removal)
• Thinking push adds at the end
• Mixing push with search or delete

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

easy

A. Node* newNode = new Node(data); newNode->next = head; head = newNode;

B. Node newNode = Node(data); head = newNode.next;

C. newNode = Node(data); newNode.next = head; head = newNode;

D. Node* newNode = Node(data); head = newNode->next;

Solution

1. Step 1: Create new node dynamically

   Use new Node(data) to allocate memory and initialize node.

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

   Set newNode->next = head and then head = newNode to push at front.

3. Final Answer:

   Node* newNode = new Node(data); newNode->next = head; head = newNode; -> Option A

4. Quick Check:

   Dynamic allocation + pointer update = B [OK]

Hint: Use pointer and 'new' keyword to create and link node [OK]

Common Mistakes:

• Not using pointer (*) for new node
• Using dot (.) instead of arrow (->) for pointers
• Assigning head incorrectly

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

struct Node {
  int data;
  Node* next;
  Node(int val) : data(val), next(nullptr) {}
};

Node* head = nullptr;

// Push 10
Node* newNode1 = new Node(10);
newNode1->next = head;
head = newNode1;

// Push 20
Node* newNode2 = new Node(20);
newNode2->next = head;
head = newNode2;

// Push 30
Node* newNode3 = new Node(30);
newNode3->next = head;
head = newNode3;

// Print list
Node* temp = head;
while(temp != nullptr) {
  std::cout << temp->data << " -> ";
  temp = temp->next;
}
std::cout << "null";

medium

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

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

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

D. null

Solution

1. Step 1: Trace push operations

   First push 10: list is 10 -> null. Then push 20: list becomes 20 -> 10 -> null. Then push 30: list becomes 30 -> 20 -> 10 -> null.

2. Step 2: Print linked list

   Printing from head prints 30, then 20, then 10, then null.

3. Final Answer:

   30 -> 20 -> 10 -> null -> Option B

4. Quick Check:

   Push adds at head, last pushed is first = C [OK]

Hint: Last pushed node appears first in list [OK]

Common Mistakes:

• Assuming push adds at tail
• Mixing order of nodes
• Not updating head pointer correctly

4. Identify the error in this push function for a singly linked list:

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

medium

A. head is passed by value, so original list head is not updated

B. newNode is not allocated dynamically

C. newNode->next should be nullptr

D. Function should return void* instead of void

Solution

1. Step 1: Analyze parameter passing

   head is passed by value, so changes inside function do not affect original head pointer.

2. Step 2: Effect on linked list

   Assigning head = newNode only changes local copy; original list head remains unchanged.

3. Final Answer:

   head is passed by value, so original list head is not updated -> Option A

4. Quick Check:

   Pass pointer by reference or pointer to pointer [OK]

Hint: Pass head by reference or pointer to update original [OK]

Common Mistakes:

• Ignoring parameter passing method
• Not returning new head if passed by value
• Incorrect pointer usage

5. You want to implement a stack using a singly linked list with push operation. Which approach correctly maintains the stack's LIFO order?

hard

A. Push new nodes at the tail and update head pointer

B. Push new nodes at the tail of the linked list without tail pointer

C. Push new nodes at the head of the linked list

D. Insert new nodes randomly in the list

Solution

1. Step 1: Understand stack LIFO behavior

   Stack requires last-in, first-out order, so newest element must be accessed first.

2. Step 2: Push at head maintains LIFO

   Adding new nodes at head means newest node is always at front, enabling O(1) push and pop.

3. Step 3: Problems with tail insertion

   Inserting at tail without tail pointer is inefficient and does not maintain LIFO easily.

4. Final Answer:

   Push new nodes at the head of the linked list -> Option C

5. Quick Check:

   Stack push = insert at head = A [OK]

Hint: Stack push uses head insertion for O(1) LIFO [OK]

Common Mistakes:

• Pushing at tail without tail pointer (slow)
• Not maintaining LIFO order
• Inserting nodes randomly