Implement Stack Using Queue in DSA Go - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to initialize a new queue for the stack.

DSA Go

type Stack struct {
    queue []int
}

func NewStack() *Stack {
    return &Stack{queue: [1]
}

Drag options to blanks, or click blank then click option'

Amake([]int, 1)

B[]int{0}

Cnil

Dmake([]int, 0)

Attempts:

3 left

💡 Hint

Common Mistakes

Using nil slice which can cause issues when appending.

Initializing slice with length 1 which adds an unwanted element.

We use make([]int, 0) to create an empty slice to represent the queue.

2fill in blank

medium

Complete the code to push an element onto the stack using the queue.

DSA Go

func (s *Stack) Push(x int) {
    s.queue = append(s.queue, x)
    for i := 0; i < [1]; i++ {
        s.queue = append(s.queue, s.queue[0])
        s.queue = s.queue[1:]
    }
}

Drag options to blanks, or click blank then click option'

Alen(s.queue) - 1

Blen(s.queue)

Clen(s.queue) + 1

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Rotating the queue the full length causing extra rotations.

Not rotating at all, which breaks stack order.

We rotate the queue len(s.queue) - 1 times to move the newly added element to the front.

3fill in blank

hard

Fix the error in the Pop method to correctly remove and return the top element.

DSA Go

func (s *Stack) Pop() int {
    if len(s.queue) == 0 {
        return -1
    }
    top := s.queue[[1]]
    s.queue = s.queue[1:]
    return top
}

Drag options to blanks, or click blank then click option'

A1

Blen(s.queue) - 1

C0

D-1

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to pop from the end of the slice which is incorrect here.

Using an invalid index causing runtime errors.

The top element is always at the front of the queue (index 0) after rotation in Push.

4fill in blank

hard

Fill both blanks to implement the Top method that returns the top element without removing it.

DSA Go

func (s *Stack) Top() int {
    if len(s.queue) == 0 {
        return -1
    }
    return s.queue[[1]] // top element
}

Drag options to blanks, or click blank then click option'

A0

Blen(s.queue) - 1

C1

D-1

Attempts:

3 left

💡 Hint

Common Mistakes

Returning the last element instead of the first.

Returning -1 incorrectly when stack is not empty.

The top element is at index 0 because of the rotation done in Push.

5fill in blank

hard

Fill all three blanks to implement the IsEmpty method that checks if the stack is empty.

DSA Go

func (s *Stack) IsEmpty() bool {
    return [1] == [2]
}

// Usage example:
// stack := NewStack()
// empty := stack.IsEmpty() // returns [3] when stack is empty

Drag options to blanks, or click blank then click option'

Alen(s.queue)

B0

Ctrue

Dfalse

Attempts:

3 left

💡 Hint

Common Mistakes

Comparing queue length to 1 instead of 0.

Returning false when stack is empty.

Stack is empty when the length of the queue is zero, so len(s.queue) == 0 returns true.

Practice

1. What is the main idea behind implementing a stack using a queue?

easy

A. Use one or two queues to simulate Last-In-First-Out (LIFO) behavior

B. Use a queue to store elements in sorted order

C. Use a stack to simulate First-In-First-Out (FIFO) behavior

D. Use two stacks to implement a queue

Solution

1. Step 1: Understand stack and queue behaviors

   A stack follows Last-In-First-Out (LIFO), while a queue follows First-In-First-Out (FIFO).

2. Step 2: Simulate stack using queue(s)

   To implement a stack using queue(s), we use one or two queues to reorder elements so the last pushed element is always dequeued first.

3. Final Answer:

   Use one or two queues to simulate Last-In-First-Out (LIFO) behavior -> Option A

4. Quick Check:

   Stack = LIFO simulated by queue(s) [OK]

Hint: Remember stack is LIFO; queue is FIFO, reorder to simulate stack [OK]

Common Mistakes:

• Confusing stack and queue order
• Trying to sort elements instead of reordering
• Using stack to implement queue instead

2. Which of the following is the correct Go syntax to declare a queue using a slice of integers?

easy

A. var queue []int

B. queue := make(map[int]int)

C. queue := []string{}

D. var queue int

Solution

1. Step 1: Identify queue data structure type

   A queue can be implemented using a slice of integers for integer elements.

2. Step 2: Check Go syntax for slice declaration

   "var queue []int" declares a slice of integers, suitable for queue implementation.

3. Final Answer:

   var queue []int -> Option A

4. Quick Check:

   Slice of int = queue [OK]

Hint: Use slice []int for queue of integers in Go [OK]

Common Mistakes:

• Using map instead of slice for queue
• Declaring wrong data type for queue
• Using string slice for integer queue

3. Consider this Go code snippet implementing push operation of stack using one queue:

func (s *Stack) Push(x int) {
    s.q = append(s.q, x)
    for i := 0; i < len(s.q)-1; i++ {
        s.q = append(s.q, s.q[0])
        s.q = s.q[1:]
    }
}

If initially s.q = [1, 2], what is s.q after calling Push(3)?

medium

A. [3, 2, 1]

B. [1, 2, 3]

C. [3, 1, 2]

D. [2, 1, 3]

Solution

1. Step 1: Append new element 3 to queue

   Initial queue: [1, 2]. After append: [1, 2, 3].

2. Step 2: Rotate queue elements to move 3 to front

   Loop runs len(s.q)-1 = 2 times: - Iteration 1: append s.q[0]=1, queue becomes [1, 2, 3, 1], then remove first element: [2, 3, 1] - Iteration 2: append s.q[0]=2, queue becomes [2, 3, 1, 2], then remove first element: [3, 1, 2]

3. Final Answer:

   [3, 1, 2] -> Option C

4. Quick Check:

   Push moves new element front: [3,1,2] [OK]

Hint: New push rotates queue to front [OK]

Common Mistakes:

• Not rotating queue after append
• Removing wrong elements during rotation
• Confusing order of elements after push

4. This Go code tries to implement Pop operation for stack using one queue:

func (s *Stack) Pop() int {
    if len(s.q) == 0 {
        return -1
    }
    val := s.q[0]
    s.q = s.q[1:]
    return val
}

What is the bug in this Pop method?

medium

A. It returns -1 when queue is empty, should panic instead

B. It removes the last element instead of the first

C. It does not check if queue is nil before accessing

D. It removes the front element, but stack pop should remove the last pushed element

Solution

1. Step 1: Understand Pop operation in stack

   Pop removes the last pushed element (top of stack).

2. Step 2: Analyze given Pop code

   Code removes s.q[0], the front of queue, which is incorrect because stack top is at front only if queue is rotated properly.

3. Final Answer:

   It removes the front element, but stack pop should remove the last pushed element -> Option D

4. Quick Check:

   Pop must remove last pushed, not front [OK]

Hint: Pop must remove top, not front element [OK]

Common Mistakes:

• Removing front element without rotation
• Ignoring empty queue check
• Returning wrong default value

5. You want to implement a stack using two queues q1 and q2 in Go. Which approach correctly implements the Push operation to keep the stack order?

hard

A. Dequeue all elements from q1 to q2, then enqueue new element to q1

B. Enqueue new element to q2, then dequeue all from q1 to q2, swap q1 and q2

C. Enqueue new element to q1 only, no other operations

D. Enqueue new element to q1, then dequeue one element from q2

Solution

1. Step 1: Understand two-queue stack push logic

   To keep stack order, new element must be at front of main queue after push.

2. Step 2: Push by enqueue to empty q2, move all from q1 to q2, then swap

   Enqueue new element to q2, move all q1 elements to q2, then swap q1 and q2 so q1 has new order with new element front.

3. Final Answer:

   Enqueue new element to q2, then dequeue all from q1 to q2, swap q1 and q2 -> Option B

4. Quick Check:

   Push uses q2 enqueue + q1 transfer + swap [OK]

Hint: Push to empty q2, move q1 to q2, swap queues [OK]

Common Mistakes:

• Not swapping queues after transfer
• Enqueueing new element to wrong queue
• Forgetting to move all elements