I recommend the 250 points and 500 points problems of topcoder(http://www.topcoder.com/ http://www.topcoder.com/). Some of the classical puzzles are so straight

I recommend the 250 points and 500 points problems of topcoder(http://www.topcoder.com/ http://www.topcoder.com/). Some of the classical puzzles are so straightforward, while more effort is needed to find the tricks of the problems on topcoder.