What-are-P-NP-NP-complete-and-NP-hard

https://www.amazon.com/Computational-Complexity-Approach-Sanjeev-Arora/dp/0521424267

http://theory.cs.princeton.edu/complexity/book.pdf

http://vdisk.weibo.com/s/auqRJ0iyNhIjJ

https://cn.udacity.com/course/intro-to-theoretical-computer-science--cs313/

https://www.quora.com/What-is-polynomial-time-reduction-2

https://en.wikipedia.org/wiki/Reduction_(complexity)

https://www.quora.com/What-is-polynomial-time-reduction-2

Introduction to the theory of complexity

http://piluc.dsi.unifi.it/piluc/files/itc.pdf

https://en.wikipedia.org/wiki/NP-completeness

https://www.quora.com/If-any-problem-in-NP-can-be-reduced-to-problems-in-P-does-that-show-p-np

https://cs.stackexchange.com/questions/3439/how-to-reduce-to-an-np-hard-problem

https://cs.stackexchange.com/questions/1526/all-np-problems-reduce-to-np-complete-problems-so-how-can-np-problems-not-be-np

https://www.cs.cmu.edu/~ckingsf/bioinfo-lectures/npcomplete.pdf

https://stackoverflow.com/questions/3671429/why-are-np-problems-called-that-way-and-np-hard-and-np-complete 

https://www.quora.com/What-are-P-NP-NP-complete-and-NP-hard

https://en.wikipedia.org/wiki/P_versus_NP_problem

https://en.wikipedia.org/wiki/NP-hardness

https://www.quora.com/How-do-you-explain-NP-Complete-and-NP-hard-to-a-child

https://en.wikipedia.org/wiki/Time_complexity#Polynomial_time

https://www.quora.com/Which-is-a-better-intuition-on-P-NP-NP-Complete-or-NP-Hard

https://www.quora.com/What-is-the-difference-between-NP-hard-and-NP-complete