两个不带头结点的有序单列表la和lb合并，合并到la中，并释放lb空间

可以这样实现：

void mergeList(ListNode* &la, ListNode* &lb) {
    ListNode* p = la;
    ListNode* q = lb;
    ListNode* r = nullptr;
    if (!p) {
        la = lb;
        lb = nullptr;
        return;
    }
    if (!q) {
        return;
    }
    if (p->val <= q->val) {
        r = p;
        p = p->next;
    } else {
        r = q;
        q = q->next;
    }
    la = r;
    while (p && q) {
        if (p->val <= q->val) {
            r->next = p;
            r = p;
            p = p->next;
        } else {
            r->next = q;
            r = q;
            q = q->next;
        }
    }
    if (p) {
        r->next = p;
    } else {
        r->next = q;
        lb = nullptr;
    }
}