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Data Structures and Algorithms Stack Queue

Peek Top Operation

Lesson 40/87

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Data Structures and Algorithms Stack Queue

Peek Top Operation

Lesson 40/87

Stack • Core Operations

The Peek/Top Operation

Peeking allows you to observe the top element of the stack without actually removing it. This is a crucial read-only O(1) operation.

TimeO(1)

SpaceO(1)

Try Interactive Simulator

Step 1: Check Underflow

Always verify if `IsEmpty()` is true. If the stack is completely empty (`top === -1`), you cannot peek. In C++, this causes undefined behavior!

Step 2: Read Value

Simply access the array at the `top` index: `stack[top]`. You read the value, but you do NOT modify the array.

Step 3: Preserve Pointer

Unlike `pop()`, the `top` pointer remains exactly where it was. The stack's structural state is entirely unchanged by a peek.

▶️ Play to watch how Peek reads the Top element without altering the pointer.

Current Step

Stack contains [10, 20, 30]. Top pointer is at index 2.

SpeedTimeline

Stack (Size 5)

◀Top: 2

Peek Result

Code Tracker

:::

1function peek() {

2 if (top === -1) {

3 console.log('Stack Underflow');

4 return null;

5 }

6 return stack[top];

Loading Lab...

Stack • Core Operations

The Peek/Top Operation

Peeking allows you to observe the top element of the stack without actually removing it. This is a crucial read-only O(1) operation.

TimeO(1)

SpaceO(1)

Try Interactive Simulator

Step 1: Check Underflow

Always verify if `IsEmpty()` is true. If the stack is completely empty (`top === -1`), you cannot peek. In C++, this causes undefined behavior!

Step 2: Read Value

Simply access the array at the `top` index: `stack[top]`. You read the value, but you do NOT modify the array.

Step 3: Preserve Pointer

Unlike `pop()`, the `top` pointer remains exactly where it was. The stack's structural state is entirely unchanged by a peek.

▶️ Play to watch how Peek reads the Top element without altering the pointer.

Current Step

Stack contains [10, 20, 30]. Top pointer is at index 2.

SpeedTimeline

Stack (Size 5)

◀Top: 2

Peek Result

Code Tracker

:::

1function peek() {

2 if (top === -1) {

3 console.log('Stack Underflow');

4 return null;

5 }

6 return stack[top];