I am a Canadian citizen and had a YMS visa that expired on May 10th 2018. I left the UK on September 1st 2017. I plan on going to the UK to visit some friends at the end of November/beginning of December, will I have problems at the border?
Assuming you are doing nothing more in the UK than a short visit with friends, and unless you have some issue with UK immigration you have not told us about, you will have no problems at all.
Canadians do not need a visa to visit the UK for short visits. Revisiting one year after you have left on another visa is certainly not too short a time.