cf-698c: probabities
cf-249d: add solution
hdu5828: add solution, segment tree
hdu5937: add solution, category discussion, enumerate
bzoj1444: add solution, ac automaton and gauss
hdu5955: add solution, ac automaton and gauss
cf-464d: nice probability dp, cut the upper bound
cf-542e: nice graph theory, find some properisity
cf-209c: nice graph theory, construct and greedy
cf-632f: nice minimum spanning tree
cf-632f: nice minimum spanning tree