三要素：定义域、值域、对应关系；两函数相等看定义域与对应关系。
How many people do we need to have in a room to make it a favorable bet (probability of success greater than 1/2) tha…
一个教室里随便点 23 个人，两两生日撞上的概率刚过半。直觉猜 183，真答案是 23。
