Push Operation on Stack in DSA C++ - Execution Trace

Concept Flow - Push Operation on Stack

Create new node with value

↓

Check if stack is empty?

No→Set new node.next to current top

↓

Update top to new node

↓

Set top to new node

↓

Done

The push operation creates a new node, links it to the current top if any, then updates the top pointer to the new node.

Execution Sample

DSA C++

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

void push(Node*& top, int val) {
  Node* newNode = new Node{val, top};
  top = newNode;
}

This code adds a new element to the top of the stack by creating a new node and updating the top pointer.

Execution Table

Step	Operation	Nodes in Stack	Pointer Changes	Visual State
1	Start with empty stack	None	top = nullptr	top -> null
2	Push 10	10	newNode->data=10; newNode->next=nullptr; top=newNode	top -> [10] -> null
3	Push 20	20 -> 10	newNode->data=20; newNode->next=top(10); top=newNode	top -> [20] -> [10] -> null
4	Push 30	30 -> 20 -> 10	newNode->data=30; newNode->next=top(20); top=newNode	top -> [30] -> [20] -> [10] -> null
5	End	30 -> 20 -> 10	top points to node with 30	top -> [30] -> [20] -> [10] -> null

💡 No more push operations; stack top points to last pushed node.

Variable Tracker

Variable	Start	After Push 10	After Push 20	After Push 30
top	nullptr	Node(10)	Node(20)	Node(30)
newNode	N/A	Node(10)	Node(20)	Node(30)
stack size	0	1	2	3

Key Moments - 3 Insights

Why does newNode->next point to the current top before updating top?

Because newNode needs to link to the previous top node to keep the stack order. See execution_table step 3 where newNode->next=top(10) before top is updated.

What happens if the stack is empty when pushing the first element?

The new node's next pointer is set to nullptr since top is nullptr. This is shown in execution_table step 2 where newNode->next=nullptr.

Why do we update top to newNode after setting newNode->next?

Updating top last ensures the stack points to the new top node with correct linkage. This order is critical as shown in steps 2-4.

Visual Quiz - 3 Questions

Look at the execution_table at step 3, what does newNode->next point to?

A Node with value 10

B nullptr

C Node with value 20

D Node with value 30

💡 Hint

Check the 'Pointer Changes' and 'Nodes in Stack' columns at step 3.

At which step does the stack size become 2 according to variable_tracker?

A After Push 10

B After Push 20

C After Push 30

D Start

💡 Hint

Look at the 'stack size' row in variable_tracker.

If we push a new value 40 after step 4, what will top point to?

A Node with value 30

B Node with value 20

C Node with value 40

D Node with value 10

💡 Hint

Refer to the concept_flow and how top updates after each push.

Concept Snapshot

Push Operation on Stack:
- Create new node with value
- Set new node's next to current top
- Update top to new node
- Stack grows by adding new nodes at the top
- top always points to the last pushed node

Full Transcript

The push operation on a stack adds a new element at the top. First, a new node is created with the given value. If the stack is empty, the new node's next pointer is set to null. Otherwise, it points to the current top node. Then, the top pointer is updated to this new node. This way, the stack grows by linking new nodes on top of the previous ones. The execution table shows step-by-step how nodes are added and how pointers change. The variable tracker follows the top pointer and stack size after each push. Key moments clarify why the new node links to the old top before updating top, and what happens when the stack is empty. The visual quiz tests understanding of pointer changes and stack size during push operations.

Practice

1. What does the push operation do in a stack?

easy

A. Checks if the stack is empty

B. Removes the top element from the stack

C. Returns the bottom element of the stack

D. Adds an element to the top of the stack

Solution

1. Step 1: Understand stack behavior

   A stack follows Last In First Out (LIFO) principle, where elements are added or removed from the top.

2. Step 2: Define push operation

   The push operation adds a new element on top of the existing stack elements.

3. Final Answer:

   Adds an element to the top of the stack -> Option D

4. Quick Check:

   Push = Add top element [OK]

Hint: Push means add element on top of stack [OK]

Common Mistakes:

• Confusing push with pop operation
• Thinking push removes elements
• Assuming push checks stack emptiness

2. Which of the following is the correct syntax to push an integer 10 onto a stack named stk in C++ STL?

easy

A. stk.push(10);

B. stk.add(10);

C. stk.insert(10);

D. stk.append(10);

Solution

1. Step 1: Recall C++ STL stack methods

   The standard method to add an element to a stack is push().

2. Step 2: Match correct syntax

   Using stk.push(10); correctly pushes 10 onto the stack.

3. Final Answer:

   stk.push(10); -> Option A

4. Quick Check:

   push() is the correct method [OK]

Hint: Use push() to add elements in C++ stack [OK]

Common Mistakes:

• Using add(), insert(), or append() which are not stack methods
• Forgetting parentheses after push
• Confusing vector methods with stack

3. What will be the output after executing the following C++ code?

std::stack<int> stk;
stk.push(5);
stk.push(10);
stk.push(15);
int topElement = stk.top();

What is the value of topElement?

medium

A. 15

B. 10

C. 0

D. 5

Solution

1. Step 1: Trace push operations

   Elements pushed in order: 5, then 10, then 15. Stack top is last pushed element.

2. Step 2: Identify top element

   The top element after pushes is 15, as it was pushed last.

3. Final Answer:

   15 -> Option A

4. Quick Check:

   Last pushed = top element = 15 [OK]

Hint: Top element is last pushed value [OK]

Common Mistakes:

• Choosing first pushed element as top
• Confusing top() with pop()
• Assuming stack is empty initially

4. Identify the error in the following code snippet that tries to push an element onto a stack:

std::stack<int> stk;
stk.push 20;

medium

A. Using wrong data type for stack

B. Stack not declared properly

C. Missing parentheses in push call

D. No error in the code

Solution

1. Step 1: Check push syntax

   The push method requires parentheses around the argument, like push(20).

2. Step 2: Identify syntax error

   Code uses stk.push 20; missing parentheses, causing a syntax error.

3. Final Answer:

   Missing parentheses in push call -> Option C

4. Quick Check:

   push() needs parentheses [OK]

Hint: Always use parentheses with push() [OK]

Common Mistakes:

• Omitting parentheses after push
• Confusing push with pop syntax
• Assuming stack declaration is wrong

5. Given an empty stack, what will be the state of the stack after executing the following code?

std::stack<int> stk;
stk.push(1);
stk.push(2);
stk.push(3);
stk.pop();
stk.push(4);

Choose the correct sequence of elements from top to bottom.

hard

A. 3 -> 2 -> 1 -> null

B. 4 -> 2 -> 1 -> null

C. 1 -> 2 -> 4 -> null

D. 2 -> 4 -> 1 -> null

Solution

1. Step 1: Trace push and pop operations

   Push 1, stack: 1
   Push 2, stack: 2 -> 1
   Push 3, stack: 3 -> 2 -> 1
   Pop removes top (3), stack: 2 -> 1
   Push 4, stack: 4 -> 2 -> 1

2. Step 2: Write final stack state

   Top to bottom: 4 -> 2 -> 1 -> null

3. Final Answer:

   4 -> 2 -> 1 -> null -> Option B

4. Quick Check:

   Push/pop order = 4,2,1 [OK]

Hint: Pop removes top; next push adds new top [OK]

Common Mistakes:

• Ignoring pop effect on stack
• Assuming push adds at bottom
• Mixing order of elements after pop