蔡勒公式

int Change(int year,int month,int day)//根据日期判断出星期几
{
    if(month == 1 || month == 2) {
        month += 12;
        year--;
    }
    int c = year / 100;
    int y = year % 100;
    int m = month;
    int d = day;
    int W = c/4-2*c+y+y/4+26*(m+1)/10+d-1;//蔡勒公式 
    if(W < 0)    return (W+(-W/7+1)*7)%7;
    return W%7;
}