What algorithm can iterate over decimal numbers with repetitions?

Question

C++ / C#

It is required to enumerate all four-digit decimal numbers in which each next digit does not exceed the previous one and implement the solution in C/C++.

Answer 1

int result, i, j, k, l;
for (i=1;i<=9;i++)
    for (j=0;j<=i;j++)
        for (k=0;k<=j;k++)
            for (l=0;l<=k;l++) {
                result = i*1000 + j*100 + k*10 + l;
                cout << result << endl;
            }

Answer 2

You can make a function
and feed numbers into it in order.