Right-angled triangle pattern with recursion numbers C++

I’m trying to print a right angled triangle with ascending and descending numbers using recursion only.

void straightTriangular(int num)
{
    if (num == 0)
    {
        return;
    }

    straightTriangular(num - 1);

    for (int i = 1; i <= num; i++)
    {
        cout << i;
    }
    cout << endl;
}

How can I do this with recursion only without "for" loop?

if the user input number is 4 then
I want the output to be this:

1
121
12321
1234321

my output using the code I posted:

1
12
123
1234

>Solution :

You can have:

• a printTriangleRec function that goes on printing every line recursively,
• two printAscendingRec and printDescendingRec functions that print the two halves of each line recursively.

[Demo]

#include <iostream>  // cout

void printAscendingRec(int cur, int top)
{
    std::cout << cur;
    if (cur != top)
    {
        printAscendingRec(cur + 1, top);
    }
}

void printDescendingRec(int cur)
{
    if (cur)
    {
        std::cout << cur;
        printDescendingRec(cur - 1);
    }
}

void printTriangleRec(int cur, int top)
{
    printAscendingRec(1, cur);
    printDescendingRec(cur - 1);
    std::cout << "\n";
    if (cur != top)
    {
        printTriangleRec(cur + 1, top);
    }
}

void printTriangle(int num)
{
    if (num < 1)
    {
        std::cout << "Error: num < 1\n";
        return;
    }
    printTriangleRec(1, num);
}

int main()
{
    printTriangle(4);
}