用doolittle分解法（非选主元）解用户输入的线性方程组。若无法解，将提示用选主元的doolittle分解法(见另一上传文件)解方程-with doolittle decomposition (non-elected PCA) user input solutions of linear equations. If there is no solution, the election will be prompted to use the PCA doolittle decompositio