Peek Top Element of Stack in DSA C# - Execution Trace

Concept Flow - Peek Top Element of Stack

Start

↓

Check if stack is empty?

Yes→Return error or null

No↓

Access top element

↓

Return top element

↓

End

The flow checks if the stack is empty. If not, it accesses and returns the top element without removing it.

Execution Sample

DSA C#

Stack<int> stack = new Stack<int>();
stack.Push(10);
stack.Push(20);
int top = stack.Peek();
Console.WriteLine(top);

This code pushes two numbers onto the stack and then peeks at the top element, printing it.

Execution Table

Step	Operation	Stack State (top -> bottom)	Pointer Changes	Visual State
1	Create empty stack	empty	head = null	null
2	Push 10	10	head -> node(10)	10 -> null
3	Push 20	20 -> 10	head -> node(20)	20 -> 10 -> null
4	Peek top element	20 -> 10	head unchanged	20 -> 10 -> null
5	Print top element	20 -> 10	head unchanged	20 -> 10 -> null
6	End	20 -> 10	head unchanged	20 -> 10 -> null

💡 Peek does not remove element; stack remains unchanged after peek.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
stack	empty	10	20 -> 10	20 -> 10	20 -> 10
head pointer	null	node(10)	node(20)	node(20)	node(20)
top element (peek)	N/A	N/A	N/A	20	20

Key Moments - 3 Insights

Why does peek not remove the top element from the stack?

Peek only accesses the top element without changing the stack, as shown in steps 4 and 5 where the stack state remains '20 -> 10'.

What happens if we peek when the stack is empty?

The flow in step 1 shows a check for empty stack; if empty, peek returns an error or null instead of accessing any element.

Why does the head pointer not change during peek?

Peek only reads the top element, so the head pointer remains pointing to the same node, as seen in steps 4 and 5.

Visual Quiz - 3 Questions

Look at the execution table, what is the stack state after step 3?

A 10

B 20 -> 10

C empty

D 30 -> 20 -> 10

💡 Hint

Check the 'Stack State' column in row for step 3.

At which step does the peek operation happen?

A Step 2

B Step 3

C Step 4

D Step 5

💡 Hint

Look for the 'Operation' column mentioning 'Peek top element'.

If we peek on an empty stack, what should happen according to the concept flow?

A Return error or null

B Push a new element

C Return the top element

D Remove the bottom element

💡 Hint

Refer to the decision branch 'Check if stack is empty?' in the concept flow.

Concept Snapshot

Peek Top Element of Stack:
- Checks if stack is empty first
- Returns top element without removing it
- Stack remains unchanged after peek
- Throws error or returns null if empty
- Useful to see next item without pop

Full Transcript

This visualization shows how the peek operation on a stack works. First, the stack is checked if empty. If not, the top element is accessed and returned without removing it. The stack's internal pointer (head) does not change during peek. The example code pushes 10 and 20, then peeks to get 20. The stack state remains '20 -> 10' after peek. If the stack were empty, peek would return an error or null. This helps understand peek as a safe way to look at the top element without modifying the stack.

Practice

1. What does the Peek() method do in a stack?

easy

A. Returns the top element without removing it

B. Removes and returns the top element

C. Adds a new element to the top

D. Checks if the stack is empty

Solution

1. Step 1: Understand the Peek method purpose

   The Peek method is designed to look at the top element of the stack without changing the stack itself.

2. Step 2: Differentiate from Pop method

   Unlike Pop, Peek does not remove the element; it only returns it.

3. Final Answer:

   Returns the top element without removing it -> Option A

4. Quick Check:

   Peek() returns top element without removal [OK]

Hint: Peek shows top without removing it [OK]

Common Mistakes:

• Confusing Peek with Pop
• Thinking Peek adds elements
• Assuming Peek empties the stack

2. Which of the following is the correct syntax to peek the top element of a stack named myStack in C#?

easy

A. var top = myStack.Pop();

B. var top = myStack.Top();

C. var top = myStack.Peek();

D. var top = myStack.GetTop();

Solution

1. Step 1: Recall Stack method names in C#

   The Stack class in C# uses Peek() to look at the top element without removing it.

2. Step 2: Verify method correctness

   Top(), GetTop() are not valid Stack methods; Pop() removes the element.

3. Final Answer:

   var top = myStack.Peek(); -> Option C

4. Quick Check:

   Peek() is correct syntax for top element [OK]

Hint: Use Peek() method to get top element [OK]

Common Mistakes:

• Using Pop() instead of Peek()
• Using non-existent methods like Top()
• Missing parentheses after Peek

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

Stack<int> numbers = new Stack<int>();
numbers.Push(10);
numbers.Push(20);
numbers.Push(30);
Console.WriteLine(numbers.Peek());
Console.WriteLine(numbers.Count);

medium

A. 10
3

B. 30
3

C. 20
2

D. 30
2

Solution

1. Step 1: Trace the Push operations

   Elements pushed in order: 10, then 20, then 30. Stack top is 30.

2. Step 2: Evaluate Peek and Count

   Peek returns top element 30 without removing it. Count remains 3 as no Pop occurred.

3. Final Answer:

   30 3 -> Option B

4. Quick Check:

   Peek = 30, Count = 3 [OK]

Hint: Peek shows last pushed; Count unchanged by Peek [OK]

Common Mistakes:

• Assuming Peek removes element
• Confusing Count after Peek
• Mixing order of pushed elements

4. Identify the error in this code snippet that tries to peek the top element of a stack:

Stack<string> stack = new Stack<string>();
string top = stack.Peek;
Console.WriteLine(top);

medium

A. Peek removes the element, so top is null

B. Stack is not initialized properly

C. Console.WriteLine cannot print stack elements

D. Peek is used without parentheses, causing a syntax error

Solution

1. Step 1: Check method call syntax

   Peek is a method and must be called with parentheses: Peek()

2. Step 2: Identify error type

   Using Peek without parentheses causes a compile-time syntax error.

3. Final Answer:

   Peek is used without parentheses, causing a syntax error -> Option D

4. Quick Check:

   Method calls need () [OK]

Hint: Always use parentheses when calling methods [OK]

Common Mistakes:

• Forgetting parentheses on Peek
• Assuming Peek removes element
• Thinking Console.WriteLine can't print strings

5. Given a stack Stack<char> letters with elements 'a', 'b', 'c' pushed in that order, write the output after this code:

char top1 = letters.Peek();
letters.Pop();
char top2 = letters.Peek();
Console.WriteLine(top1);
Console.WriteLine(top2);
Console.WriteLine(letters.Count);

hard

A. c
b
2

B. a
b
3

C. c
c
3

D. b
a
1

Solution

1. Step 1: Trace initial stack and first Peek

   Stack after pushes: bottom 'a', then 'b', top 'c'. Peek() returns 'c' as top1.

2. Step 2: Pop removes top element 'c'

   After Pop(), top 'c' is removed. Stack top is now 'b'.

3. Step 3: Second Peek returns 'b' as top2

   Peek() now returns 'b'. Count is 2 because one element was removed.

4. Final Answer:

   c b 2 -> Option A

5. Quick Check:

   Peek then Pop then Peek updates top correctly [OK]

Hint: Peek shows current top; Pop removes it; next Peek shows new top [OK]

Common Mistakes:

• Assuming Peek removes element
• Confusing order of elements after Pop
• Miscounting stack size after Pop