I need to know the real life or real time examples of above mentioned problems thanks!!
closed as not constructive by Yannis Rizos♦ Apr 19 '12 at 11:18
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P: testing whether a number is prime. (See: http://www.jstor.org/stable/10.2307/3597229)
NP: integer factoring because it is easy to check whether two numbers multiply to a desired number. (Actually any P problem is also in NP.)
NP-hard: traveling salesman (See: http://en.wikipedia.org/wiki/Traveling_salesman_problem)
NP-complete: decision version of traveling salesman
Take a look at the answers and comments here on this SO question ...
There's some interesting book and website links there.