扩展欧几里德算法不但能计算(a,b)的最大公约数，而且能计算a模b及b模a的乘法逆元。平方和乘算法的复杂度：执行次数至少要k次模乘，最多需要２ｋ次模乘。-Extended Euclidean algorithm for computing not only (a, b) the common denominator, and to calculate a modulus b and b modulus a multiplicative inverse of. Square and by the