void insertbefore(linklist &l, int i, int e)
{
    if (i < 1)
    {
        // i小于1时输入非法直接返回
        return;
    }
    lnode *pre = l;
    for (int j = 1; j < i; j++)
    {
        if (pre)
        {
            pre = pre->next;
        }
    }
    lnode *s = new lnode;
    s->data = e;
    s->next = pre->next;
    pre->next = s;
}