摘要：
题面： If we represent a date in the format YYYY-MM-DD (for example, 2017-04-09), do you know how many 9s will appear in all the dates between Y1-M1-D1 a 阅读全文
摘要：
题面： There are n courses in the course selection system of Marjar University. The i-th course is described by two values: happiness Hi and credit Ci. I 阅读全文