O(1)快速乘

求两个数相乘超过long long取摸的快速运算O(1)  

inline long long multi(long long x,long long y,long long mod)
{
long long tmp=(x*y-(long long)((long double)x/mod*y+1.0e-8)*mod);
return tmp<0 ? tmp+mod : tmp;
}