DATA STRUCTURES PROGRAM • QUEUES & DEQUES

Reverse a Queue Using a Stack

Learn how to reverse a queue using a stack using a clear C program.

BeginnerQueueFIFOfront/rear

PROBLEM UNDERSTANDING

Input and expected output

Sample input

No input required

Sample output

40 30 20 10

COMPLETE C PROGRAM

Complete C implementation

#include <stdio.h>

int main(void)
{
    int queue[] = {10, 20, 30, 40};
    int stack[4], top = -1;
    for (int front = 0; front < 4; front++) stack[++top] = queue[front];
    for (int rear = 0; rear < 4; rear++) queue[rear] = stack[top--];
    for (int index = 0; index < 4; index++) printf("%d ", queue[index]);
    putchar('\n');
    return 0;
}

CURRENT STEP

Select Start to walk through the important lines.

SELECTED LINE

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EXPECTED OUTPUT FOR THE SAMPLE

40 30 20 10

0%Step 0 of 0

PROGRAM EXPLANATION

Algorithm and explanation

Read the required input values.
Move every queue item through a LIFO stack before enqueuing it again.
Display the computed result.

Move every queue item through a LIFO stack before enqueuing it again.

EFFICIENCY

Time and space complexity

Time complexity

O(n)

Auxiliary space

O(n)

DEBUGGING CHECKLIST

Common mistakes

Check this

Use the correct format specifier for every variable.

Check this

Initialize variables before using their values.

Check this

Check braces, semicolons and input order carefully.

Try it yourself

Practice: Run the program with the sample input, predict its output, and then test one boundary case of your own.