高频题（1）
1. 树模型分叉标准
解释一下tree model在训练的时候是怎么分叉的，判别的条件是什么。
2. 聚类
在coderpad上写clustering的class
3. 游戏金币设计
在游戏里的金币出现通货膨胀问题，大家的金币越来越多，不希望有这个问题，怎么解决。
4. 硬币生成随机数
一枚硬币怎么生成7个随机数。
5. Coin Change
You are given an integer array coins representing coins of different denominations and an integer amount representing a total amount of money.
Return the fewest number of coins that you need to make up that amount. If that amount of money cannot be made up by any combination of the coins, return -1.
You may assume that you have an infinite number of each kind of coin.
