Reverse a Number in C++ using Recursion.

Given a number num, write a C++ Program to reverse the given number using Recursion.

Example:
Input: num = 1234
Output: 4321

Input: num = -345
Output: -543

To solve this problem, you must have some prior knowledge of C++ programming and Recursion.

Follow the steps given below to reverse a number using Recursion.

In the reverse function, we first check if the number is zero. If it is, we return from the function, as there's nothing to reverse.
If the number is not zero, we output the last digit of the number using the modulo operator (%).
We then call the reverse function again, passing the number divided by 10, to remove the last digit.
This process continues until the number becomes zero.

C++ code to Reverse a Positive Number.

//C++ Program to reverse positive number using Recursion
#include <iostream>
using namespace std;

//function call
void reverse(int num) {
    if (num == 0) {
        return;
    }

    cout << num % 10;
    reverse(num / 10);
}

int main() {
    int num;
    cout << "Enter a number: ";
    cin >> num;

    cout << "The reverse of the number is: ";
    reverse(num);
    cout << endl;
    return 0;
}

Output:

Enter a number: 4738
The reverse of the number is: 8374

Time Complexity: O(log(n))
Auxiliary Space: O(log(n))

Note: The above algorithm can reverse only positive numbers so to reverse both positive and negative numbers, we have made some changes in the above algorithm. (alert-passed)

Reverse a Negative Number in C++

Following steps to reverse both positive and negative numbers using recursion:

If the number is not zero, we output the last digit of the number using the abs function and modulo operator (%).
We then call the reverse function again, passing the absolute value of the number divided by 10, to remove the last digit.
This process continues until the number becomes zero.
We read the input number and check if it's negative. If it is, we output a negative sign before calling the reverse function.

C++ code to reverse both positive and negative numbers.

//C++ Program to reverse both positive and negative num
#include <iostream>
using namespace std;

//function call
void reverse(int num) {
    if (num == 0) {
        return;
    }

    cout << abs(num % 10);
    reverse(num / 10);
}

int main() {
    int num;
    cout << "Enter a number: ";
    cin >> num;

    cout << "The reverse of the number is: ";
    //checking if num is negative
    if (num < 0) {
        cout << "-";
    }
    
    reverse(abs(num));
    cout << endl;
    return 0;
}

Output:

Enter a number: -14738
The reverse of the number is: -83741

Time Complexity: O(log(n))
Auxiliary Space: O(log(n))

Reverse a Number in C++ using Recursion.

Reverse a Negative Number in C++

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