I was watching this YouTube video on "P vs NP" https://www.youtube.com/watch?v=YX40hbAHx3s .
In this video, they mention how some problems that were originally thought to be NP - a "trick" was then discovered such that these problems could be reclassified as P.
I tried searching online for concrete examples of this - for example, some specific math/computer problems which were NP and the precise "trick" that was discovered to make them as P.
But I don't think I was correctly searching the necessary keywords to retrieve such examples as I was not able to find specific examples.
Could someone please tell me of some specific examples of problems that were originally considered as NP and what lead them to be reclassified as P?
Thanks!