Queue Using Two Stacks in DSA C - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to push an element onto the first stack.

DSA C

void enqueue(int x) {
    stack1[top1++] = [1];
}

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

Atop1

Bstack2[top2]

Cx

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong variable instead of x.

Trying to push onto stack2 instead of stack1.

We push the value x onto stack1 at the current top1 index.

2fill in blank

medium

Complete the code to check if both stacks are empty (queue is empty).

DSA C

int isEmpty() {
    return (top1 == 0 && [1] == 0);
}

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

Astack2

Btop1

Cstack1

Dtop2

Attempts:

3 left

💡 Hint

Common Mistakes

Checking stack arrays instead of top indices.

Using top1 twice instead of top1 and top2.

The queue is empty if both stack1 and stack2 are empty, so top2 must be zero.

3fill in blank

hard

Fix the error in the dequeue function to correctly transfer elements from stack1 to stack2.

DSA C

int dequeue() {
    if (top2 == 0) {
        while (top1 > 0) {
            stack2[top2++] = [1];
            top1--;
        }
    }
    return stack2[--top2];
}

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

Astack1[top1 - 1]

Bstack2[top2 - 1]

Cstack2[top2]

Dstack1[top1]

Attempts:

3 left

💡 Hint

Common Mistakes

Using top1 instead of top1-1 causes out-of-bounds access.

Copying from stack2 instead of stack1.

We move the top element from stack1 (at top1-1) to stack2 and then decrease top1.

4fill in blank

hard

Fill both blanks to correctly initialize the queue and reset stack tops.

DSA C

void initQueue() {
    top1 = [1];
    top2 = [2];
}

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

A0

B-1

C1

DNULL

Attempts:

3 left

💡 Hint

Common Mistakes

Setting top indices to -1 which is not used in this implementation.

Using NULL which is not valid for integer indices.

Both top1 and top2 start at 0 indicating empty stacks.

5fill in blank

hard

Fill all three blanks to correctly check if the queue is empty and return an error value if so.

DSA C

int dequeue() {
    if (top1 == 0 && top2 == 0) {
        return [1];
    }
    if (top2 == 0) {
        while (top1 > 0) {
            stack2[top2++] = stack1[[2]];
            top1--;
        }
    }
    return stack2[[3]];
}

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

A-1

Btop1 - 1

Ctop2 - 1

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Returning 0 instead of -1 for empty queue.

Using top1 or top2 directly instead of top-1 for indexing.

Return -1 if empty; move from stack1[top1-1]; return stack2[top2-1].

Practice

1. What is the main idea behind implementing a Queue Using Two Stacks?

easy

A. Use two stacks to store elements in sorted order

B. Use two stacks to reverse the order twice to maintain queue order

C. Use one stack for enqueue and another for dequeue operations

D. Use two stacks to perform only enqueue operations

Solution

1. Step 1: Understand queue order

   A queue follows FIFO (First In First Out) order, meaning the first element added is the first to be removed.

2. Step 2: Use two stacks to reverse order twice

   One stack reverses the order when pushing elements, and the second stack reverses it again when popping, restoring FIFO order.

3. Final Answer:

   Use two stacks to reverse the order twice to maintain queue order -> Option B

4. Quick Check:

   Two reversals keep FIFO order [OK]

Hint: Two stacks reverse order twice to simulate queue [OK]

Common Mistakes:

• Thinking both stacks store elements in sorted order
• Using stacks only for enqueue or dequeue, not both
• Confusing stack order with queue order

2. Which of the following is the correct syntax to push an element x onto a stack named stack1 in C?

easy

A. push(stack1, x);

B. stack1.push(x); // C syntax

C. stack1.push(x);

D. push(x, stack1);

Solution

1. Step 1: Recall stack push function in C

   In C, stacks are usually implemented with functions like push(stack, element).

2. Step 2: Identify correct argument order

   The stack is passed first, then the element to push, so push(stack1, x) is correct.

3. Final Answer:

   push(stack1, x); -> Option A

4. Quick Check:

   Function call with stack first, element second [OK]

Hint: In C, push(stack, element) is standard [OK]

Common Mistakes:

• Using object.method() syntax which is not valid in C
• Swapping argument order in push function
• Assuming push is a built-in method

3. Consider the following code snippet implementing a queue using two stacks s1 and s2. What will be the output after the operations?

push(s1, 10);
push(s1, 20);
pop(s2);
if (isEmpty(s2)) {
  while (!isEmpty(s1)) {
    push(s2, top(s1));
pop(s1);
  }
}
pop(s2);
printQueue(s1, s2);

medium

A. Queue elements: 10 -> null

B. Queue elements: null

C. Queue elements: 20 -> null

D. Queue elements: 10 -> 20 -> null

Solution

1. Step 1: Push 10 and 20 onto s1

   s1 = [10 (bottom), 20 (top)], s2 = []

2. Step 2: Pop from s2 (empty), then transfer all from s1 to s2

   Transfer reverses order: s2 = [20 (bottom), 10 (top)], s1 = []

3. Step 3: Pop from s2 removes 10 (top)

   s2 = [20]

4. Step 4: printQueue shows elements in queue order

   Queue order is s2 top to bottom then s1 bottom to top, so only 20 remains.

5. Final Answer:

   Queue elements: 20 -> null -> Option C

6. Quick Check:

   First popped element was 10, next is 20 [OK]

Hint: Transfer reverses stack order to simulate queue dequeue [OK]

Common Mistakes:

• Not transferring elements from s1 to s2 when s2 is empty
• Confusing top and bottom elements during transfer
• Assuming pop removes from s1 directly

4. The following code attempts to implement dequeue operation in a queue using two stacks s1 and s2. What is the bug?

int dequeue() {
  if (isEmpty(s2)) {
    while (!isEmpty(s1)) {
      push(s2, top(s1));
      pop(s1);
    }
  }
  return pop(s2);
}

medium

A. Missing check if both stacks are empty before dequeue

B. pop(s2) returns void, should use top(s2) before pop

C. Should push from s2 to s1 instead of s1 to s2

D. pop(s1) should be called after push(s2, top(s1))

Solution

1. Step 1: Check for empty queue condition

   The code does not check if both s1 and s2 are empty before dequeue, which can cause errors.

2. Step 2: Identify missing empty check

   Without this check, pop(s2) may be called on empty stack causing runtime error.

3. Final Answer:

   Missing check if both stacks are empty before dequeue -> Option A

4. Quick Check:

   Empty queue must be handled before popping [OK]

Hint: Always check if queue is empty before dequeue [OK]

Common Mistakes:

• Assuming pop returns the popped value in C without checking
• Ignoring empty queue condition
• Confusing push direction between stacks

5. You want to implement a queue using two stacks s1 and s2. You perform the following operations:

enqueue(1);
enqueue(2);
dequeue();
enqueue(3);
dequeue();
dequeue();

What will be the output sequence of the dequeued elements?

hard

A. [3, 2, 1]

B. [1, 3, 2]

C. [2, 1, 3]

D. [1, 2, 3]

Solution

1. Step 1: Enqueue 1 and 2

   s1 = [1 (bottom), 2 (top)], s2 = []

2. Step 2: Dequeue first element

   s2 empty, transfer all from s1 to s2: s2 = [2 (bottom), 1 (top)], s1 = []
   Pop from s2 removes 1

3. Step 3: Enqueue 3

   Push 3 to s1: s1 = [3], s2 = [2]

4. Step 4: Dequeue second element

   s2 not empty, pop from s2 removes 2

5. Step 5: Dequeue third element

   s2 empty, transfer s1 to s2: s2 = [3], s1 = []
   Pop from s2 removes 3

6. Final Answer:

   [1, 2, 3] -> Option D

7. Quick Check:

   FIFO order maintained by two stacks [OK]

Hint: Dequeue triggers transfer only if second stack empty [OK]

Common Mistakes:

• Not transferring elements when s2 is empty
• Assuming enqueue adds to s2 instead of s1
• Mixing order of popped elements