Queue vs Stack When to Use Which in DSA C - Compare & Choose

Challenge - 5 Problems

🧠 Conceptual

intermediate

Choosing Between Queue and Stack for Task Scheduling

You have a list of tasks that must be processed in the exact order they arrive. Which data structure is best to use?

A Stack, because it processes the most recent task first.

B Queue, because it processes tasks in reverse order.

C Stack, because it allows random access to tasks.

D Queue, because it processes tasks in the order they arrive.

💡 Hint

Think about which data structure follows the 'first come, first served' rule.

A queue processes elements in the order they arrive (FIFO), which is perfect for task scheduling where order matters. A stack processes elements in reverse order (LIFO), which is not suitable here.

❓ Predict Output

intermediate

Output of Stack Operations

What is the printed output after executing the following stack operations?

DSA C

Stack s = create_stack();
push(s, 10);
push(s, 20);
push(s, 30);
int x = pop(s);
print(x);
int y = pop(s);
print(y);

A

30
20

B

10
20

C

20
30

D

10
30

💡 Hint

Remember that stack removes the last pushed item first.

Stack is LIFO. The last pushed item is 30, so pop returns 30 first, then 20.

❓ Predict Output

advanced

Queue Behavior with Enqueue and Dequeue

What is the state of the queue after these operations?

DSA C

Queue q = create_queue();
enqueue(q, 5);
enqueue(q, 10);
dequeue(q);
enqueue(q, 15);
print_queue(q);

A 5 -> 10 -> 15 -> null

B 10 -> 15 -> null

C 15 -> 10 -> null

D 5 -> 15 -> null

💡 Hint

Remember that dequeue removes from the front of the queue.

After enqueue 5 and 10, queue is 5 -> 10. Dequeue removes 5. Then enqueue 15 adds to the end. Final queue is 10 -> 15.

🔧 Debug

advanced

Identifying Error in Stack Usage

What error will this code produce?

DSA C

Stack s = create_stack();
push(s, 1);
pop(s);
pop(s);

A Underflow error because pop is called on empty stack.

B Overflow error because stack is full.

C No error, returns 1 then 0.

D Syntax error due to missing semicolon.

💡 Hint

Think about what happens when you pop more times than you push.

After popping once, the stack is empty. Popping again causes underflow error because there is nothing to pop.

🧠 Conceptual

expert

Choosing Data Structure for Undo Feature

You are designing an undo feature for a text editor. Which data structure is best suited and why?

A Queue, because it allows random access to previous actions.

B Queue, because it processes actions in the order they were done.

C Stack, because it allows reversing the most recent action first.

D Stack, because it processes actions in the order they were done.

💡 Hint

Undo means reversing the last action first.

An undo feature needs to reverse the most recent action first, which is exactly how a stack works (LIFO). A queue processes in the order actions were done, which is not suitable.

Practice

1. Which data structure should you use when you want to process tasks in the exact order they arrive, like customers waiting in line at a store?

easy

A. Queue, because it processes elements in First-In-First-Out order

B. Stack, because it processes elements in Last-In-First-Out order

C. Array, because it allows random access

D. Tree, because it organizes data hierarchically

Solution

1. Step 1: Understand the order of processing tasks

   Tasks arriving in order and processed in the same order means First-In-First-Out (FIFO).

2. Step 2: Match FIFO with the correct data structure

   Queue follows FIFO, so it fits the scenario of customers waiting in line.

3. Final Answer:

   Queue, because it processes elements in First-In-First-Out order -> Option A

4. Quick Check:

   FIFO = Queue [OK]

Hint: FIFO means use Queue, like a line of customers [OK]

Common Mistakes:

• Confusing FIFO with LIFO
• Choosing Stack for ordered processing
• Thinking arrays maintain insertion order automatically

2. Which of the following is the correct way to push an element onto a stack in C?

easy

A. push(element, stack);

B. stack.push(element);

C. stack->push(element);

D. push(stack, element);

Solution

1. Step 1: Recall typical C stack function syntax

   In C, stack operations are usually functions taking the stack and element as arguments.

2. Step 2: Identify correct function call order

   push(stack, element) is the standard way to push an element onto a stack in C.

3. Final Answer:

   push(stack, element); -> Option D

4. Quick Check:

   Function call with stack first = push(stack, element) [OK]

Hint: In C, function calls usually have data structure first [OK]

Common Mistakes:

• Using object.method() syntax which is not C style
• Swapping argument order in function call
• Assuming stack is a pointer with methods

3. What will be the output of the following C code snippet using a stack?

#include 
#include 

#define MAX 5

typedef struct {
    int items[MAX];
    int top;
} Stack;

void push(Stack *s, int val) {
    if (s->top == MAX - 1) return;
    s->items[++s->top] = val;
}

int pop(Stack *s) {
    if (s->top == -1) return -1;
    return s->items[s->top--];
}

int main() {
    Stack s;
    s.top = -1;
    push(&s, 10);
    push(&s, 20);
    push(&s, 30);
    printf("%d ", pop(&s));
    printf("%d ", pop(&s));
    return 0;
}

medium

A. 10 20

B. 30 20

C. 20 10

D. 30 10

Solution

1. Step 1: Trace push operations on stack

   Push 10, stack: [10]; push 20, stack: [10,20]; push 30, stack: [10,20,30]. Top points to 30.

2. Step 2: Trace pop operations and output

   First pop returns 30, second pop returns 20, so output is "30 20 ".

3. Final Answer:

   30 20 -> Option B

4. Quick Check:

   LIFO pop order = 30 then 20 [OK]

Hint: Stack pops last pushed first (LIFO) [OK]

Common Mistakes:

• Assuming FIFO order for stack
• Mixing push and pop order
• Ignoring initial top = -1

4. The following C code tries to implement a queue but has a bug. What is the main issue?

#include 
#define MAX 3

int queue[MAX];
int front = -1, rear = -1;

void enqueue(int val) {
    if (rear == MAX - 1) {
        printf("Queue full\n");
        return;
    }
    queue[++rear] = val;
    if (front == -1) front = 0;
}

int dequeue() {
    if (front == -1 || front > rear) {
        printf("Queue empty\n");
        return -1;
    }
    return queue[front++];
}

int main() {
    enqueue(1);
    enqueue(2);
    enqueue(3);
    dequeue();
    enqueue(4);
    printf("%d\n", queue[rear]);
    return 0;
}

medium

A. The enqueue function uses wrong index for insertion

B. The dequeue function returns wrong element

C. The queue does not reset front and rear after dequeueing all elements, causing overflow

D. The queue size is too large

Solution

1. Step 1: Understand enqueue and dequeue behavior

   Enqueue adds at rear, dequeue removes from front. After dequeueing all, front > rear but variables not reset.

2. Step 2: Identify problem with overflow after dequeue

   After dequeueing, rear stays at max index, so enqueue thinks queue is full even if space is free.

3. Final Answer:

   The queue does not reset front and rear after dequeueing all elements, causing overflow -> Option C

4. Quick Check:

   Queue pointers not reset = overflow bug [OK]

Hint: Reset front and rear when queue becomes empty [OK]

Common Mistakes:

• Ignoring front and rear reset after emptying queue
• Thinking enqueue index is wrong
• Assuming dequeue returns wrong element

5. You need to design a system that supports undo and redo operations for text editing. Which data structure(s) should you use and why?

hard

A. Two stacks: one for undo and one for redo, because stacks support LIFO order needed for undo/redo

B. One queue, because it processes commands in order

C. One stack only, because redo is not needed

D. Two queues, because they maintain order of commands

Solution

1. Step 1: Understand undo/redo operation order

   Undo reverses last action (LIFO), redo re-applies undone actions in reverse order.

2. Step 2: Choose data structures matching LIFO behavior

   Two stacks allow pushing actions for undo and redo separately, supporting LIFO order for both.

3. Final Answer:

   Two stacks: one for undo and one for redo, because stacks support LIFO order needed for undo/redo -> Option A

4. Quick Check:

   Undo/Redo = Two stacks (LIFO) [OK]

Hint: Undo/Redo needs two stacks for LIFO operations [OK]

Common Mistakes:

• Using queues which are FIFO, not suitable for undo
• Ignoring redo functionality
• Using only one stack which can't handle redo