Stack Implementation Using Array in DSA Javascript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a stack in data structures?

A stack is a collection where elements are added and removed from only one end, called the top. It follows Last In, First Out (LIFO) order.

beginner

What does the 'push' operation do in a stack?

The 'push' operation adds a new element to the top of the stack.

beginner

What does the 'pop' operation do in a stack?

The 'pop' operation removes the top element from the stack and returns it.

intermediate

How can an array be used to implement a stack?

An array can store stack elements where the last element in the array is the top of the stack. Push adds to the end, pop removes from the end.

beginner

What happens if you try to pop from an empty stack?

Trying to pop from an empty stack usually returns undefined or throws an error, because there is no element to remove.

Which order does a stack follow?

A Sorted order

B First In, First Out (FIFO)

C Random order

D Last In, First Out (LIFO)

A stack always follows Last In, First Out (LIFO) order.

In a stack implemented with an array, where is the top element located?

A At the start of the array

B At the end of the array

C In the middle of the array

D At a random position

The top element is at the end of the array for easy push and pop operations.

What does the 'peek' operation do in a stack?

A Returns the top element without removing it

B Adds a new element

C Removes the top element

D Clears the stack

'Peek' returns the top element without removing it from the stack.

What will happen if you push an element onto a full stack implemented using a fixed-size array?

A The element is added successfully

B The bottom element is removed

C Stack overflow error occurs

D The stack resets

Pushing onto a full fixed-size stack causes a stack overflow error.

Which JavaScript array method is commonly used to implement the 'pop' operation in a stack?

A pop()

B push()

C shift()

D unshift()

The pop() method removes the last element from an array, matching stack's pop operation.

Explain how to implement a stack using an array in JavaScript, including push and pop operations.

Think about how array methods push() and pop() work.

Describe what happens when you try to pop from an empty stack and how to handle it.

Consider edge cases in stack operations.

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