中文说明：此代码虽简单却不失深刻，通用性极强。我认为是作为一个最初学习最小二乘法MATLAB应用的新手应该学习的一个代码。此代码分为三个部分：线性拟合、多项式拟合与非线性拟合。全面！实用！
English Description：
Although this code is simple, it is very general. I think it is a code that should be learned by a novice who initially studied the application of least squares in MATLAB. This code is divided into three parts: linear fitting, polynomial fitting and nonlinear fitting. Comprehensive! practical!