Canadian travelling from Japan to Shanghai

I am Canadian. I presume that a flight from Japan to Shanghai and back to Japan does not qualify me for the 72/144h twov exemption? However, am I correct that it would be acceptable for me to go from Japan to Shanghai to Korea then back to Japan in order to get the 72/144h exemption?
With regards to that, how long must my stay in the next country be for that to qualify?