Queue vs Stack When to Use Which in DSA C - Complexity Comparison

Time Complexity: Queue vs Stack When to Use Which

O(n)

Understanding Time Complexity

We want to understand how the time it takes to use a queue or a stack changes as we add more items.

Which one is faster or better for certain tasks?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

// Simple stack push and pop
void stackOperations(int n) {
    int stack[n];
    int top = -1;
    for (int i = 0; i < n; i++) {
        stack[++top] = i; // push operation
    }
    while (top >= 0) {
        int val = stack[top--]; // pop operation
    }
}

// Simple queue enqueue and dequeue
void queueOperations(int n) {
    int queue[n];
    int front = 0, rear = -1;
    for (int i = 0; i < n; i++) {
        queue[++rear] = i; // enqueue operation
    }
    while (front <= rear) {
        int val = queue[front++]; // dequeue operation
    }
}
    

This code shows basic push/pop for stack and enqueue/dequeue for queue with n items.

Identify Repeating Operations

Both stack and queue perform repeated operations:

• Primary operation: push and pop for stack; enqueue and dequeue for queue.
• How many times: Each operation runs exactly n times, once per item.

How Execution Grows With Input

As the number of items n grows, the total operations grow in a straight line.

Input Size (n)	Approx. Operations
10	20 (10 pushes + 10 pops)
100	200 (100 pushes + 100 pops)
1000	2000 (1000 pushes + 1000 pops)

Pattern observation: Operations increase directly with n, doubling because each item is added and then removed.

Final Time Complexity

Time Complexity: O(n)

This means the time to process all items grows in a straight line with the number of items.

Common Mistake

[X] Wrong: "Stacks or queues take longer time as they get bigger because of complex operations inside."

[OK] Correct: Both stack and queue operations here are simple and happen once per item, so time grows linearly, not more.

Interview Connect

Understanding when to use stack or queue and their time costs helps you choose the right tool for problems and explain your choices clearly.

Self-Check

"What if we used a linked list instead of an array for the queue? How would the time complexity change?"

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