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;
}