3.4.1 closed fences
4.1.2 fence rails
4.1.4 cryptcowgraphy
4.2.4 cowcycles
4.3.2 the primes
5.2.2 electric fences（不是 3.4 那个 可以在 SPOJ 中找到）
5.2.3 wisconsin squares
5.4.1 all latin squares
5.4.3 character recognition
5.4.4 betsy`s tour
6.1.1 postal vans
6.1.2 a rectangular barn
6.1.3 cow xor