中文说明：poisson问题有限元求解通用程序(线性元).压缩包内exp.m包含了一个实际问题，matlab下直接运行即可，如效率不高，可适当修改网格参数h.
English Description：
The general program (linear element) is used to solve Poisson problem by finite element method. Exp. m in the package contains a practical problem, which can be run directly in MATLAB. If the efficiency is not high, the mesh parameter h can be modified appropriately