积累一下快读的写法
比scanf快一点点

template <typename T>
T read()
{
    T sum = 0, fl = 1;
    int ch = getchar();
    for (; !isdigit(ch); ch = getchar())
        if (ch == '-')
            fl = -1;
    for (; isdigit(ch); ch = getchar())
        sum = sum * 10 + ch - '0';
    return sum;
}

引用：

n = read<LL>();