Implement Stack Using Queue in DSA Java - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to add an element to the stack using a queue.

DSA Java

public void push(int x) {
    queue.[1](x);
}

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

Aoffer

Badd

Cremove

Dpoll

Attempts:

3 left

💡 Hint

Common Mistakes

Using remove() or poll() which remove elements instead of adding.

Using add() which can throw exceptions if the queue is full.

The offer() method adds an element to the queue. It is the correct way to enqueue an element.

2fill in blank

medium

Complete the code to remove the front element from the queue.

DSA Java

int front = queue.[1]();

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

Aadd

Bpoll

Coffer

Dpeek

Attempts:

3 left

💡 Hint

Common Mistakes

Using peek() which only returns but does not remove the element.

Using offer() or add() which add elements instead of removing.

poll() removes and returns the head of the queue, which is needed to simulate stack pop.

3fill in blank

hard

Fix the error in the code to rotate the queue elements after pushing.

DSA Java

for (int i = 0; i < queue.size() - 1; i++) {
    queue.[1](queue.[2]());
}

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

Apoll

Bpeek

Coffer

Dremove

Attempts:

3 left

💡 Hint

Common Mistakes

Using peek() which does not remove the element.

Using remove() which throws exception if queue is empty.

poll() removes the front element, and offer() adds it back. Using poll() twice would remove two elements incorrectly.

4fill in blank

hard

Fill both blanks to correctly implement the pop method using the queue.

DSA Java

public int pop() {
    return queue.[1]();
}

public int top() {
    return queue.[2]();
}

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

Apoll

Bpeek

Coffer

Dremove

Attempts:

3 left

💡 Hint

Common Mistakes

Using peek() in pop() which does not remove the element.

Using poll() in top() which removes the element.

pop() removes and returns the front element using poll(), while top() returns the front element without removing it using peek().

5fill in blank

hard

Fill all three blanks to complete the push method that rotates the queue to simulate stack push.

DSA Java

public void push(int x) {
    queue.[1](x);
    int size = queue.[2]();
    for (int i = 0; i < size - 1; i++) {
        queue.[3](queue.poll());
    }
}

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

Aoffer

Bsize

Dadd

Attempts:

3 left

💡 Hint

Common Mistakes

Using add() instead of offer() which can throw exceptions.

Using incorrect method to get size or rotate the queue.

offer() adds the element, size() gets the queue size, and offer() adds the polled element back to rotate the queue.

Practice

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

easy

A. Use one queue and reorder elements to simulate stack behavior

B. Use two stacks to simulate a queue

C. Use a linked list to implement both stack and queue

D. Use an array to simulate queue operations

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

   By reordering elements in a single queue after each push, we can simulate LIFO behavior of a stack.

3. Final Answer:

   Use one queue and reorder elements to simulate stack behavior -> Option A

4. Quick Check:

   Stack via queue = reorder in one queue [OK]

Hint: Remember stack is LIFO; reorder queue to mimic it [OK]

Common Mistakes:

• Confusing stack with queue behavior
• Using two stacks instead of queues
• Ignoring element order after push

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

easy

A. LinkedList<Integer> queue = new Queue<>();

B. Queue<int> queue = new LinkedList<>();

C. Queue<Integer> queue = new LinkedList<>();

D. Queue<Integer> queue = new Queue<>();

Solution

1. Step 1: Use interface Queue with LinkedList implementation

   In Java, Queue is an interface and LinkedList implements it. Use generics with wrapper class Integer.

2. Step 2: Check syntax correctness

   Queue<Integer> queue = new LinkedList<>(); is correct syntax for declaring a queue of integers.

3. Final Answer:

   Queue<Integer> queue = new LinkedList<>(); -> Option C

4. Quick Check:

   Queue<Integer> with LinkedList = correct syntax [OK]

Hint: Use wrapper classes and interface for queue declaration [OK]

Common Mistakes:

• Using primitive int instead of Integer
• Trying to instantiate interface Queue directly
• Confusing LinkedList and Queue syntax

3. What will be the output of the following code snippet implementing stack push using one queue?

Queue<Integer> q = new LinkedList<>();
// Push 1
q.add(1);
// Push 2
q.add(2);
// Reorder queue to simulate stack push
for (int i = 0; i < q.size() - 1; i++) {
  q.add(q.remove());
}
System.out.println(q.remove());

medium

A. 1

B. 2

C. Queue is empty

D. Runtime error

Solution

1. Step 1: Add elements 1 and 2 to queue

   Queue after adds: front -> 1, 2 -> rear

2. Step 2: Reorder queue to simulate stack push

   Loop runs q.size()-1 = 1 time: remove 1, add 1 back. Queue becomes front -> 2, 1 -> rear

3. Step 3: Remove and print element

   q.remove() removes front element 2, which is printed.

4. Final Answer:

   2 -> Option B

5. Quick Check:

   Reordered queue front = last pushed = 2 [OK]

Hint: After push reorder, front is last pushed element [OK]

Common Mistakes:

• Not reordering queue after push
• Printing wrong element without remove
• Confusing queue front with rear

4. Identify the error in this stack pop implementation using one queue:

public int pop() {
  if (queue.isEmpty()) return -1;
  return queue.remove();
}

medium

A. No error, code works correctly

B. Queue should be cleared after pop

C. Should use queue.peek() instead of remove()

D. Pop removes the oldest element, not the last pushed

Solution

1. Step 1: Understand queue remove() behavior

   queue.remove() removes the front element, which is the oldest inserted, not the last pushed.

2. Step 2: Check stack pop requirement

   Stack pop must remove the last pushed element (LIFO), but this code removes FIFO element.

3. Final Answer:

   Pop removes the oldest element, not the last pushed -> Option D

4. Quick Check:

   Queue remove() = FIFO, stack pop = LIFO [OK]

Hint: Pop must remove last pushed, not queue front [OK]

Common Mistakes:

• Using queue.remove() directly for pop
• Ignoring element order after push
• Not reordering queue to simulate stack

5. You want to implement a stack using two queues. Which sequence of operations correctly simulates the pop() method?

hard

A. Move all but last element from queue1 to queue2, remove last from queue1, swap queues

B. Remove front element from queue1 directly

C. Add element to queue2, then remove from queue1

D. Swap queues without removing any element

Solution

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

   To pop last pushed element, move all except last from queue1 to queue2, then remove last from queue1.

2. Step 2: Swap queues for next operations

   After removal, swap queue1 and queue2 references to maintain stack state.

3. Final Answer:

   Move all but last element from queue1 to queue2, remove last from queue1, swap queues -> Option A

4. Quick Check:

   Pop with two queues = move all but last, remove last, swap [OK]

Hint: Pop = move all but last, remove last, then swap queues [OK]

Common Mistakes:

• Removing front element directly
• Not swapping queues after pop
• Adding elements to wrong queue during pop