Queue Using Two Stacks in DSA C - Build from Scratch

Queue Using Two Stacks

📖 Scenario: Imagine you are managing a line of customers waiting for service. You want to keep track of who comes first and who leaves first, just like a queue. But you only have two stacks (like two piles of plates) to help you. Your task is to build a queue using these two stacks.

🎯 Goal: Build a queue using two stacks in C. You will create two stacks, add elements to the queue, remove elements from the queue, and finally print the queue's state.

📋 What You'll Learn

Create two stacks using arrays and integer variables for top positions.

Create a function to enqueue (add) elements to the queue using the first stack.

Create a function to dequeue (remove) elements from the queue using both stacks.

Print the queue elements in order after all operations.

💡 Why This Matters

🌍 Real World

Queues are used in many real-life situations like customer service lines, printer job management, and task scheduling. Using two stacks to build a queue helps understand how data structures can work together.

💼 Career

Understanding how to implement one data structure using others is a common interview question and helps improve problem-solving skills for software development roles.

1

Create Two Stacks

Create two integer arrays called stack1 and stack2 each with size 5. Also create two integer variables called top1 and top2 and set both to -1.

DSA C

# Your code here

Hint

Think of top1 and top2 as the positions of the top plates in each stack. Starting at -1 means the stacks are empty.

2

Create Enqueue Function

Write a function called enqueue that takes an integer value. Inside, increase top1 by 1 and add value to stack1[top1].

DSA C

int stack1[5];
int stack2[5];
int top1 = -1;
int top2 = -1;

// Your code here

Hint

Adding to the queue means pushing onto stack1. Increase top1 first, then put the value on top.

3

Create Dequeue Function

Write a function called dequeue that returns an integer. If top2 is -1, move all elements from stack1 to stack2 by popping from stack1 and pushing to stack2. Then pop and return the top element from stack2.

DSA C

int stack1[5];
int stack2[5];
int top1 = -1;
int top2 = -1;

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

// Your code here

Hint

To remove from the queue, if stack2 is empty, move all items from stack1 to stack2 to reverse their order. Then pop from stack2.

4

Print Queue Elements

Write code to enqueue the values 10, 20, and 30. Then dequeue one element. Finally, print the remaining elements in the queue in order by dequeuing until empty. Use printf to print each element followed by a space.

DSA C

int stack1[5];
int stack2[5];
int top1 = -1;
int top2 = -1;

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

int dequeue() {
    if (top2 == -1) {
        while (top1 != -1) {
            top2++;
            stack2[top2] = stack1[top1];
            top1--;
        }
    }
    if (top2 == -1) {
        return -1; // Queue is empty
    }
    int value = stack2[top2];
    top2--;
    return value;
}

int main() {
    // Enqueue 10, 20, 30
    // Dequeue one element
    // Print remaining elements
    // Your code here
    return 0;
}

Hint

After removing one element, the queue should have 20 and 30 left. Print them separated by spaces.

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