https://www.quora.com/What-are-P-NP-NP-complete-and-NP-hard
https://www.quora.com/How-do-you-explain-NP-Complete-and-NP-hard-to-a-child
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
What-are-P-NP-NP-complete-and-NP-hard
原文：http://www.cnblogs.com/WCFGROUP/p/7107437.html