C++ : Printing patterns

It may be a bad question for you , but I have a genuine doubt , save your downvotes.

I came across this interesting pattern :

1

2   6

3   7  10 

4   8  11  13

5   9  12  14  15

Seems quite easy , but I am not getting it.

My code:

int k = 1;
for (int n = 5 ; n>0 ; n--)
{
   for (int x = 1 ; x<=n ; x++)
      cout<<k++;
    cout<<endl;
}

So the output:

12345
6789
101112
1314
15

You can see that I have achieved a way to get the numbers right , but I am not sure how to get the pattern