先贴几个博客，帮助理解一下
1. http://trinkle.is-programmer.com/2015/6/30/polynomial-interpolation.100064.html
2. http://blog.163.com/kevinlee_2010/blog/static/16982082020141213187464/
3. http://blog.csdn.net/jiangnanyouzi/article/details/5698122
Lagrange插值式子
例题： bzoj2655 calc http://www.cnblogs.com/keshuqi/p/6659397.html