求最大公约数（欧几里得算法）

原理见百度百科：欧几里得算法

int gcd(int a, int b){if(a < b)swap(a, b);return b == 0 ? a : gcd(b, a % b);}

用于编程珠玑第二章的向量旋转问题，重新写这个程序：

#include <iostream>#include <string>using namespace std;void rotate(string &str, int i){int strLen = str.size();int numOfLoop = gcd(strLen, i);for(int loop = 0; loop < numOfLoop; ++loop){char tmp = str[loop];int current = loop;int next = loop + i;while(next % strLen != loop){str[current] = str[next];current = (current + i) % strLen;next = (next + i) % strLen;}str[current] = tmp;}}int main() {string str = "abcdefgh";rotate(str, 4);for(auto c : str)cout << c;cout << endl;return 0;}