合并有序链表

利用哑节点+三个指针：

//
// Created by jt on 2020/9/26.
//
class Solution {
public:
    /**
     *
     * @param l1 ListNode类
     * @param l2 ListNode类
     * @return ListNode类
     */
    ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
        // write code here
        ListNode dummy(0);
        ListNode *p = &dummy;
        while (l1 && l2) {
            if (l1->val > l2->val) { p->next = l2; l2 = l2->next; }
            else { p->next = l1; l1 = l1->next; }
            p = p->next;
        }
        while (l1) { p->next = l1; l1 = l1->next; p = p->next; }
        while (l2) { p->next = l2; l2 = l2->next; p = p->next; }
        p->next = nullptr;
        return dummy.next;
    }
};