快速冪

http://baike.baidu.com/link?url=bsfNWRDMEBcTrOQDx7rfWPnwa7wqzzzNBW72_ooKnLsftubiH-CLoOIy_yaNkrINBGEqCnakdM93u0Xrf-zf9K

int pow3(int x,int n)
{
    if(n==0) return 1;
    else
    {
        while((n&1)==0)
        {
            n>>=1;
            x*=x;
        }
    }
    int result=x;
    n>>=1;
    while(n!=0)
    {
        x*=x;
    if((n&1)!=0)
        result*=x;
    n>>=1;
    }
    return result;
}