Stack Implementation Using Array in DSA Javascript - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of the stack after these operations?

Consider a stack implemented using an array with the following operations:

push(10), push(20), pop(), push(30), push(40), pop()

What is the printed state of the stack (top to bottom) after these operations?

DSA Javascript

class Stack {
  constructor() {
    this.items = [];
  }
  push(element) {
    this.items.push(element);
  }
  pop() {
    if (this.items.length === 0) return null;
    return this.items.pop();
  }
  print() {
    return this.items.slice().reverse().join(' -> ') + ' -> null';
  }
}

const stack = new Stack();
stack.push(10);
stack.push(20);
stack.pop();
stack.push(30);
stack.push(40);
stack.pop();
console.log(stack.print());

A 30 -> 10 -> null

B 20 -> 10 -> null

C 10 -> 30 -> null

D 40 -> 30 -> 10 -> null

💡 Hint

Remember pop removes the top element from the stack.

After pushing 10 and 20, stack is [10,20]. Pop removes 20. Then push 30 and 40, stack is [10,30,40]. Pop removes 40. Final stack is [10,30]. Printed top to bottom is 30 -> 10 -> null.

🧠 Conceptual

intermediate

Which operation is NOT typical for a stack?

Stacks usually support push, pop, and peek operations. Which of the following operations is NOT a standard stack operation?

A push(element)

B pop()

C peek()

D enqueue(element)

💡 Hint

Think about which operation is typical for queues, not stacks.

enqueue is an operation used in queues to add elements at the end, not in stacks.

🔧 Debug

advanced

What error does this stack implementation cause?

Consider this stack pop method:

pop() {
return this.items.pop();
}

What error or issue can occur if pop is called on an empty stack?

DSA Javascript

class Stack {
  constructor() {
    this.items = [];
  }
  pop() {
    return this.items.pop();
  }
}

A Throws TypeError

B Throws RangeError

C Returns undefined without error

D Throws ReferenceError

💡 Hint

What does JavaScript's array pop() return when empty?

Array pop() returns undefined if the array is empty; no error is thrown.

❓ Predict Output

advanced

What is the output after pushing and popping multiple elements?

Given the following code, what is the printed stack state (top to bottom)?

DSA Javascript

class Stack {
  constructor() {
    this.items = [];
  }
  push(element) {
    this.items.push(element);
  }
  pop() {
    if (this.items.length === 0) return null;
    return this.items.pop();
  }
  print() {
    return this.items.slice().reverse().join(' -> ') + ' -> null';
  }
}

const stack = new Stack();
stack.push(5);
stack.push(15);
stack.push(25);
stack.pop();
stack.push(35);
stack.pop();
stack.pop();
console.log(stack.print());

A 35 -> 15 -> 5 -> null

B 5 -> null

C 25 -> 15 -> 5 -> null

D 15 -> 5 -> null

💡 Hint

Track each push and pop carefully.

Push 5,15,25: stack=[5,15,25]. Pop removes 25: stack=[5,15]. Push 35: stack=[5,15,35]. Pop removes 35: stack=[5,15]. Pop removes 15: stack=[5]. Printed top to bottom: 5 -> null.

🚀 Application

expert

How many items remain after this sequence of stack operations?

A stack starts empty. The following operations occur:

push(1), push(2), push(3), pop(), push(4), pop(), pop(), push(5)

How many items are in the stack at the end?

A 2

B 3

C 1

D 4

💡 Hint

Count pushes and pops step by step.

Operations: push 1 [1], push 2 [1,2], push 3 [1,2,3], pop removes 3 [1,2], push 4 [1,2,4], pop removes 4 [1,2], pop removes 2 [1], push 5 [1,5]. Final stack has 2 items.

Practice

1. What is the main characteristic of a stack data structure implemented using an array?

easy

A. Sorted Order

B. Last In, First Out (LIFO)

C. Random Access

D. First In, First Out (FIFO)

Solution

1. Step 1: Understand stack behavior

   A stack adds and removes elements from the same end, so the last element added is the first to be removed.

2. Step 2: Identify the correct term

   This behavior is called Last In, First Out (LIFO).

3. Final Answer:

   Last In, First Out (LIFO) -> Option B

4. Quick Check:

   Stack = LIFO [OK]

Hint: Stack always removes the last added item first [OK]

Common Mistakes:

• Confusing stack with queue (FIFO)
• Thinking stack allows random access
• Assuming stack keeps elements sorted

2. Which of the following is the correct way to add an element to a stack implemented as an array in JavaScript?

easy

A. stack.push(element);

B. stack.pop(element);

C. stack.shift(element);

D. stack.unshift(element);

Solution

1. Step 1: Identify method to add element at the end

   In JavaScript arrays, push() adds an element to the end of the array.

2. Step 2: Confirm stack push operation

   Stack push means adding element on top, which corresponds to array's end, so push() is correct.

3. Final Answer:

   stack.push(element); -> Option A

4. Quick Check:

   Use push() to add element to stack [OK]

Hint: Use push() to add on top of stack [OK]

Common Mistakes:

• Using pop() which removes element
• Using shift() or unshift() which affect start of array

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

const stack = [];
stack.push(10);
stack.push(20);
stack.pop();
stack.push(30);
console.log(stack);

medium

A. [10, 20, 30]

B. [30]

C. [20, 30]

D. [10, 30]

Solution

1. Step 1: Trace push and pop operations

   Initially stack = [] push(10) -> [10] push(20) -> [10, 20] pop() removes last element 20 -> [10] push(30) -> [10, 30]

2. Step 2: Check final stack state

   After all operations, stack contains [10, 30].

3. Final Answer:

   [10, 30] -> Option D

4. Quick Check:

   Push and pop update stack correctly [OK]

Hint: Pop removes last pushed, push adds at end [OK]

Common Mistakes:

• Assuming pop removes first element
• Confusing order of elements after pop

4. Identify the error in the following stack implementation code and select the correct fix:

class Stack {
  constructor() {
    this.items = [];
  }
  push(element) {
    this.items.unshift(element);
  }
  pop() {
    return this.items.pop();
  }
}

medium

A. Use push() instead of unshift() in push method

B. Use shift() instead of pop() in pop method

C. No error, code is correct

D. Use pop() instead of push() in push method

Solution

1. Step 1: Analyze push method

   push uses unshift which adds element at start of array.

2. Step 2: Analyze pop method

   pop removes element from end of array. This causes mismatch because push adds at start but pop removes from end.

3. Step 3: Fix mismatch

   To maintain stack behavior, push should add at end using push(), so pop removes last added element correctly.

4. Final Answer:

   Use push() instead of unshift() in push method -> Option A

5. Quick Check:

   push() and pop() must match ends [OK]

Hint: Push and pop must operate on same array end [OK]

Common Mistakes:

• Mixing unshift with pop causes wrong stack order
• Thinking shift() and pop() are interchangeable
• Ignoring array end consistency

5. You want to implement a stack using an array that also tracks the maximum element at any time efficiently. Which approach is best?

hard

A. Scan entire array to find max after each pop

B. Sort the array after each push to find max

C. Use two stacks: one for elements, one for max values

D. Store max element only in a variable updated on push

Solution

1. Step 1: Understand max tracking challenge

   Finding max after each operation must be efficient, not scanning entire array.

2. Step 2: Evaluate options

   Sorting after each push is costly. Scanning array after pop is slow. Storing max in variable fails if max is popped.

3. Step 3: Use two stacks approach

   One stack stores elements, second stack stores current max at each push. This allows O(1) max retrieval.

4. Final Answer:

   Use two stacks: one for elements, one for max values -> Option C

5. Quick Check:

   Two stacks track max efficiently [OK]

Hint: Use auxiliary stack to track max values [OK]

Common Mistakes:

• Sorting array each time is inefficient
• Single variable max fails when max is popped
• Scanning array after pop is slow