Push Using Linked List Node in DSA C++

Mental Model

Adding a new item at the start of a linked list means making it the new first item and linking it to the old first item.

Analogy: Imagine a line of people holding hands. To add a new person at the front, the new person grabs the hand of the first person, becoming the new leader of the line.

head -> 1 -> 2 -> 3 -> null

Dry Run Walkthrough

Input: list: 1 -> 2 -> 3 -> null, push value 0

Goal: Add a new node with value 0 at the start of the list

Step 1: Create new node with value 0

newNode(0) null

Why: We need a new node to add to the list

Step 2: Set new node's next to current head (node 1)

newNode(0) -> 1 -> 2 -> 3 -> null

Why: Link new node to the existing list so it points to the old first node

Step 3: Update head to new node

head -> 0 -> 1 -> 2 -> 3 -> null

Why: Make new node the first node in the list

Result:

head -> 0 -> 1 -> 2 -> 3 -> null

Annotated Code

DSA C++

#include <iostream>
using namespace std;

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

class LinkedList {
public:
    Node* head;
    LinkedList() : head(nullptr) {}

    void push(int val) {
        Node* newNode = new Node(val); // create new node
        newNode->next = head;          // link new node to current head
        head = newNode;                // update head to new node
    }

    void printList() {
        Node* curr = head;
        while (curr != nullptr) {
            cout << curr->data << " -> ";
            curr = curr->next;
        }
        cout << "null" << endl;
    }
};

int main() {
    LinkedList list;
    list.push(3);
    list.push(2);
    list.push(1);
    list.printList();

    list.push(0); // push value 0 at front
    list.printList();

    return 0;
}

Node* newNode = new Node(val); // create new node

create a new node with the given value

newNode->next = head; // link new node to current head

point new node's next to current head to link list

head = newNode; // update head to new node

make new node the new head of the list

Output

1 -> 2 -> 3 -> null

0 -> 1 -> 2 -> 3 -> null

Complexity Analysis

Time: O(1) because we only change a few pointers without traversing the list

Space: O(1) because we only create one new node regardless of list size

vs Alternative: Compared to adding at the end which requires O(n) to traverse, pushing at front is faster and simpler

Edge Cases

empty list (head is null)

new node becomes the head and points to null

DSA C++

newNode->next = head;          // link new node to current head

When to Use This Pattern

When you need to add items quickly at the start of a list, use push on linked list nodes because it avoids traversing the whole list.

Common Mistakes

Mistake: Forgetting to update the head pointer to the new node
Fix: Always assign head = newNode after linking newNode->next to old head

Mistake: Setting newNode->next to nullptr instead of head
Fix: Set newNode->next = head to keep the existing list connected

Summary

Adds a new node at the start of a linked list by linking it to the current head and updating head.

Use when you want to insert elements quickly at the front of a list.

The new node points to the old first node, then becomes the new head.

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