在实现了系数矩阵的三角分解后，为了避免“小主元”做除数而导致更大的误差，在doolittle分解的同时，每一步先选主元，再进行分解计算。-achieved in the triangular matrix decomposition, in order to avoid "small PCA" do Divisor and lead to greater error, the Doolittle decomposition at the same time, every step the first choice PCA, further decomposition.