摘要：
Cubes You are given 12 rods of equal length. Each of them is colored in certain color. Your task is to determine in how many different ways one can co 阅读全文
摘要：
He's Circles He's Circles He wrote n letters "X" and "E" in a circle. He thought that there were 2n possibilities to do it, because each letter may be 阅读全文