欧几里德算法：辗转求余　　原理: gcd(a,b)=gcd(b,a mod b)　　当b为0时,两数的最大公约数即为a　　getchar()会接受前一个scanf的回车符-Euclidean algorithm: Principles of travel for more than: gcd (a, b) = gcd (b, a mod b) when b is 0, the two common denominator is the number of a getchar () will accept the previous scanf carriage return