[Algorithm cornerstone] P problem, NP problem, NPC problem, NP-Hard problem

https://blog.csdn.net/bitcarmanlee/article/details/51935400
https://blog.csdn.net/qq_23947237/article/details/80250899
https://blog.csdn.net/huang1024rui/article/details/49154507
https://www.jianshu.com/p/a350e563d9a9