Peek Top Element of Stack in DSA C# - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the Peek operation do in a stack?

The Peek operation returns the top element of the stack without removing it. It lets you see the last added item while keeping the stack unchanged.

beginner

In C#, which method is used to look at the top element of a Stack<T> without removing it?

The method is Peek(). It returns the top item of the stack without changing the stack's contents.

intermediate

What happens if you call Peek() on an empty stack in C#?

Calling Peek() on an empty stack throws an InvalidOperationException because there is no element to look at.

beginner

Why is Peek() useful compared to Pop() in stack operations?

Peek() lets you check the top element without removing it, so you can decide what to do next. Pop() removes the top element, changing the stack.

beginner

Show a simple C# code snippet to peek the top element of a stack of integers.

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

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

A Returns the top element without removing it

B Removes the bottom element

C Returns and removes the top element

D Clears the stack

Peek() returns the top element without removing it, letting you see the last added item.

What exception is thrown if Peek() is called on an empty stack in C#?

A InvalidOperationException

B IndexOutOfRangeException

C ArgumentException

D NullReferenceException

Calling Peek() on an empty stack throws InvalidOperationException.

Which method removes the top element from a stack?

A Clear()

B Peek()

C Push()

D Pop()

Pop() removes and returns the top element of the stack.

If a stack has elements 5, 10, 15 (15 on top), what does Peek() return?

A 5

B 10

C 15

D Stack is empty

Peek() returns the top element, which is 15.

What is the difference between Peek() and Pop()?

A <code>Peek()</code> removes the element, <code>Pop()</code> does not

B <code>Pop()</code> removes the element, <code>Peek()</code> does not

C Both remove the element

D Both do not remove the element

Pop() removes the top element; Peek() only returns it without removal.

Explain how the Peek() operation works on a stack and why it might be useful.

Think about looking at the last added item without taking it out.

Describe what happens if you call Peek() on an empty stack in C# and how to handle it.

Consider safety checks before peeking.

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