trace("-1%4",-1%4);
trace("0%4",0%4);
trace("3%4",3%4);
trace("5%4",5%4);
trace("9%4",9%4);
trace("15%4",15%4);
trace("64&4",64%4);
trace("-1&4",-1&(4-1));
trace("0&4",0&(4-1));
trace("3&4",3&(4-1));
trace("5&4",5&(4-1));
trace("9&4",9&(4-1));
trace("15&4",15&(4-1));
trace("64&4",64&(4-1));
//a & (n - 1) 等价于 a % n，是取模的典型做法。
