UpsolvesCow Coupons 的贪心算法正确性证明一个贪心算法的正确性证明。UpsolvesPair XORAnother typical pair-counting problem.Upsolvesarc133_d Range XORA typical pair-counting problem.Upsolvescf_1621d The Winter HikeAn interesting problem.Upsolvesabc235_g Painting Weighted GraphA typical instance of tree DP of merge.Upsolvesabc227_g Divisors of Binomial CoefficientFactorize factorial.Upsolvesabc225_f String CardsA strict weak ordering on nonempty strings.Upsolvesabc214_g Three Permutations一个计数问题。