This problem can be solved without random exirsices :)
I have got AC just by writing triads in order(without spaces):
aaa aab aac aad... aba abb abc abd... zzz. I did it recursivly. And then you must repeat this sequence until number of characters=10^6;

Good

thank you very much

I did the same and was actually surprised when it worked! I have got no idea why would people have problems with this one, it's really easy to solve