传说中的O(lgn)时间的快速算术算法和超大整数的取模算法。
1.快速求积,a*b=a*2*b/2
int fast_mul(int a, int b){ int m = 0; while(b){ if(b & 0x01){ //a*b = a+a(b-1) m += a; --b; }else{ //a*b = a*2*b/2 a <<= 1; b >>= 1; } } return m; }
2.快速求幂,a^e=a^(2*e/2)
int fast_exp(int a, int e){ int exp = 1; while(e){ if(e & 0x01){ //a^e = a*a^(e-1) exp *= a; --e; }else{ //a^e = a^(2*e/2) a *= a; e >>= 1; } } return exp; }
3.大整数取模,(a*b)%m = ((a%m)*(b%m))%m
int bigint_mod(int a, int n, int m){ int mod = 1; while(n){ if(n & 0x01){ //(a*b)%m = ((a%m)*b)%m = (a*(b%m))%m = ((a%m)*(b%m))%m mod = (mod*a)%m; } a = (a*a)%m; n >>= 1; } return mod; }