Threads Problem in Programming Multithreading

Hi guys ;
I'm trying to solve as much as possible multithreading problems in regard of getting a job ; I've
seen this problem of multithreading:
Top Multithreading Interview Question #full Answer - YouTube


In the video explains the problem with complete solution but I didn't really understand how printf will print those possibilities:
(in the video mentioned that printf maybe not atomic)


2 1
1 1
2 2
1 2


could anyone please help me to illustrate in simple words the explanation that's found on the video? I already seen
the video but Im confused on his explanation and didn't understand how do I solve correctly with understandings that problem that's
found on the video (two threads racing over each other on print function).


Doesn't matter for me what's the language about ; I need to understand the concept of concurrency and multithreading - Im new on this world and just started to study it recently.


thanks.

Also here -> Multithreading Programming Problem - C++ Forum