Problem: Write a program that calculates the factorial of a given number using a recursive function.

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#include <iostream> unsigned long long factorial(int n) {    if (n == 0 || n == 1) {        return 1;    } else {        return n * factorial(n - 1);    } } int main() {    int num;    std::cout << "Enter a number: ";    std::cin >> num;    unsigned long long result = factorial(num);    std::cout << "The factorial of " << num << " is: " << result << std::endl;    return 0; }

In this example, we define a recursive function called factorial that calculates the factorial of a given number n. If n is 0 or 1, the function returns 1 (base case). Otherwise, it calls itself with the argument n-1 and multiplies the result by n (recursive case).

In the main function, we prompt the user to enter a number and store it in the num variable. We then call the factorial function, passing num as an argument. The return value is stored in the result variable.

Finally, we print the factorial of the number.