用全选主元Gauss消去法求解线性方程组。其中a是方程组的系数矩阵，b是右端常数向量，并存放最终解向量，n是阶数。-With full pivoting Gauss elimination method for solving linear equations. Where a is the coefficient matrix, b is the right end of the constant vector, and store the final solution vector, n is the order.