5/10/2016
1. Add about 20 more problems.
2. Optimize solutions for some old problems.
5/17/2016
1. Add 12 more problems.
2. Optimize solutions for some old problems.
3. Remove some naive solutions because they are too obvious and have no values.
8/1/2016
1. Add 30 more problems.
2. Optimize solutions for some old problems.
3. Reorder the problems and make similar problems to be together.
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8/1/2016
5/17/2016
5/10/2016
5/1/2016
3/19/2016
5/1/2015
12/1/2012