程序代码：

list<int>::iterator mid(list<int>::iterator beg, list<int>::iterator end){
    while(beg!=end){
        end--;
        if(beg==end)
            return beg;
        beg++;
    }
    if(beg==end)
        return beg;
}

list<int>::iterator mid(list<int>::iterator beg, list<int>::iterator end){
    return beg + (end - beg)/2;
}

程序代码：

std::list<int>::iterator mid( std::list<int>::iterator beg, std::list<int>::iterator end )
{
    std::list<int>::difference_type dif = distance( beg, end );
    std::advance( beg, dif/2 );
    return beg;
}

const_iterator的也一样