Implement Stack Using Queue in DSA Go - Execution Trace

Concept Flow - Implement Stack Using Queue

Start Push

↓

Enqueue new element to queue

↓

Rotate queue elements to move new element to front

↓

Push done

↓

Start Pop

↓

Dequeue front element from queue

↓

Pop done

Push adds element by enqueue and rotates queue to front. Pop removes front element from queue.

Execution Sample

DSA Go

queue := []int{}
// Push 1
queue = append(queue, 1)
// Rotate queue
// Pop
front := queue[0]
queue = queue[1:]

This code pushes 1 by enqueue and rotates queue, then pops front element.

Execution Table

Step	Operation	Queue State Before	Pointer Changes	Queue State After	Visual State
1	Push 1	[]	Enqueue 1	[1]	1 -> null
2	Rotate after Push 1	[1]	No rotation needed	[1]	1 -> null
3	Push 2	[1]	Enqueue 2	[1, 2]	1 -> 2 -> null
4	Rotate after Push 2	[1, 2]	Dequeue 1, Enqueue 1	[2, 1]	2 -> 1 -> null
5	Push 3	[2, 1]	Enqueue 3	[2, 1, 3]	2 -> 1 -> 3 -> null
6	Rotate after Push 3	[2, 1, 3]	Dequeue 2, Enqueue 2; Dequeue 1, Enqueue 1	[3, 2, 1]	3 -> 2 -> 1 -> null
7	Pop	[3, 2, 1]	Dequeue 3	[2, 1]	2 -> 1 -> null
8	Pop	[2, 1]	Dequeue 2	[1]	1 -> null
9	Pop	[1]	Dequeue 1	[]	empty

💡 All pushes and pops completed; queue empty after last pop.

Variable Tracker

Variable	Start	After Step 1	After Step 4	After Step 6	After Step 7	After Step 8	After Step 9
queue	[]	[1]	[2, 1]	[3, 2, 1]	[2, 1]	[1]	[]

Key Moments - 3 Insights

Why do we rotate the queue after each push?

Rotating moves the newly added element to the front, so pop can remove it first, simulating stack's LIFO. See steps 3-6 in execution_table.

Why does pop remove from the front of the queue?

Because after rotation, the newest element is at the front, so dequeue removes the last pushed element, matching stack pop. See steps 7-9.

What happens if we pop when the queue is empty?

There is no element to dequeue, so pop should handle empty queue safely. This example ends with empty queue at step 9.

Visual Quiz - 3 Questions

Look at the execution_table, what is the queue state after step 6?

A [3, 2, 1]

B [1, 2, 3]

C [2, 1, 3]

D [1, 3, 2]

💡 Hint

Check the 'Queue State After' column at step 6 in execution_table.

At which step does the first pop operation remove element 3?

A Step 5

B Step 7

C Step 6

D Step 8

💡 Hint

Look at the 'Operation' and 'Pointer Changes' columns in execution_table for pop steps.

If we skip rotation after push, what would be the queue state after pushing 2?

A [2, 1]

B [1]

C [1, 2]

D [2]

💡 Hint

Without rotation, new elements are added at the end; see step 3 before rotation.

Concept Snapshot

Implement Stack Using Queue:
- Use one queue.
- Push: enqueue new element, then rotate queue to front.
- Pop: dequeue front element.
- Rotation moves newest element to front for LIFO.
- Simulates stack with queue operations.

Full Transcript

This concept shows how to build a stack using a single queue. When pushing, we add the new element to the end of the queue, then rotate all previous elements behind it so the new element moves to the front. This way, popping simply removes the front element, which is the last pushed item, matching stack behavior. The execution table traces each push and pop step, showing queue states and pointer changes. Key moments clarify why rotation is needed and how pop works. The visual quiz tests understanding of queue states and operations during the process.

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