比如单元格 A1=5，然后 A2=A1+2，A3=A1+A2，然后这时修改 A1=A3-5，excel 会提示单元格存在循环依赖错误
如果用 ruby 代码，该怎么写？把 A[N] 设为数组或者哈希吧
linked list cycle https://leetcode.com/problems/linked-list-cycle/
http://tomasp.net/blog/2018/write-your-own-excel/ 里面有
