使用遞歸,a*b = a+a+a+...(b個a)
int mul(int a, int b){
if (a == 0 || b == 0)
return 0;
if (b == 1)
return a;
if (a == 1)
return b;
return a + mul(a, b - 1);
}
int mult(int a,int b){
int m = mul(a, abs(b));
return (b<0)?(-m):m;
}