之前看过一个给出[0, m) ,求[0, n)的随机数
static int getRandom(int n, int m) {
int i = 1;
while((n*i-1)%(m-1) != 0) i++;
int factor = (n*i-1)/(m-1);
int randomSum = 0;
while(factor > 0){
randomSum += rand_m();
factor--;
}
return randomSum%n;
}