Peek Top Element of Stack in DSA C - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of peeking the top element in this stack?

Consider a stack implemented using an array with the following operations. What will be printed after peeking the top element?

DSA C

#include <stdio.h>
#define MAX 5

int stack[MAX];
int top = -1;

void push(int x) {
    if (top < MAX - 1) {
        stack[++top] = x;
    }
}

int peek() {
    if (top == -1) return -1;
    return stack[top];
}

int main() {
    push(10);
    push(20);
    push(30);
    printf("%d\n", peek());
    return 0;
}

A 10

B 20

C 30

D -1

💡 Hint

Peek returns the element at the top index of the stack array.

The stack has elements pushed in order: 10, 20, 30. The top points to the last pushed element, which is 30. So peek returns 30.

❓ Predict Output

intermediate

What happens when peeking an empty stack?

Given the stack implementation below, what will be the output when peek is called on an empty stack?

DSA C

#include <stdio.h>
#define MAX 3

int stack[MAX];
int top = -1;

int peek() {
    if (top == -1) return -1;
    return stack[top];
}

int main() {
    printf("%d\n", peek());
    return 0;
}

A -1

B 0

C Garbage value

D Runtime error

💡 Hint

Check the condition when top is -1 in peek function.

When the stack is empty, top is -1. The peek function returns -1 in this case, indicating no elements.

🔧 Debug

advanced

Identify the error in this peek function implementation

The peek function below is intended to return the top element of the stack. What error will occur when this code runs?

DSA C

int peek() {
    if (top == 0) return -1;
    return stack[top];
}

A Returns wrong value when stack has one element

B Returns -1 correctly when stack is empty

C Causes segmentation fault

D Compiles but returns garbage value

💡 Hint

Check the condition that checks if stack is empty.

The condition 'top == 0' incorrectly treats stack with one element as empty. The empty stack condition is 'top == -1'. So peek returns -1 even when one element exists.

❓ Predict Output

advanced

What is the output after multiple push and peek operations?

Given the following code, what will be printed?

DSA C

#include <stdio.h>
#define MAX 4

int stack[MAX];
int top = -1;

void push(int x) {
    if (top < MAX - 1) {
        stack[++top] = x;
    }
}

int peek() {
    if (top == -1) return -1;
    return stack[top];
}

int main() {
    push(5);
    push(15);
    printf("%d\n", peek());
    push(25);
    push(35);
    printf("%d\n", peek());
    return 0;
}

A

25
35

B

5
35

C

15
25

D

15
35

💡 Hint

Peek returns the last pushed element at each call.

After pushing 5 and 15, peek returns 15. After pushing 25 and 35, peek returns 35.

🧠 Conceptual

expert

What is the time complexity of the peek operation in a stack?

Consider a stack implemented using an array or linked list. What is the time complexity of the peek operation?

A O(n^2) - Quadratic time

B O(1) - Constant time

C O(log n) - Logarithmic time

D O(n) - Linear time

💡 Hint

Peek only accesses the top element without iteration.

Peek returns the top element directly using the top index or pointer, so it runs in constant time O(1).