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