N皇后问题代码求讲解

void test(long row, long ld, long rd)
{
    if (row != upperlim)   
    {
        long pos = upperlim & ~(row | ld | rd);
        while (pos)  
        {   
            long p = pos & -pos;
            pos -= p;
            if(row==0&&p>half)
                continue;
            else
                test(row + p, (ld + p) << 1, (rd + p) >> 1);   
        }
    }
    else   
    {
        sum++;   
    }
}
int main(int argc, char *argv[])
{
    time_t tm;
    int n;
    printf("请输入皇后个数 N [ 0<n<32 ]:");
    scanf("%d",&n);
    if (argc != 1)
        n = atoi(argv[1]);
    tm = time(0);
    if ((n < 1) || (n > 32))                  
    {
        printf("该程序只能计算1到32\n");
        exit(-1);
    }
    if(n==1)
    {
        sum=1;
        printf("共有%ld种放法. \n花费时间为%.12lf秒. \n", sum,  (int)(time(0) - tm));
        return 0;  
    }
    printf("完成%d皇后问题可能花费一点时间，请耐心等待...\n", n);   
    half=(n-1)/2;  
    half=upperlim<<half;   
    upperlim = (upperlim << n) - 1;
    test(0, 0, 0);
    printf("共有%ld种放法. \n花费时间为%d秒. \n", sum*2, (int)(time(0) - tm));
    return 0;
}
求解释