LL mult(LL a, LL k, LL m){ LL res = 0; while(k){ if(k & 1LL) res = (res + a) % m; k >>= 1; a = (a << 1) % m; } return res; }
二分乘法
原文：http://www.cnblogs.com/Heilce/p/6403680.html
