完全同 AC36. 合并两个排序的序列

讨论中有不同的解法（递归和迭代的简易版）

class Solution {
public:
    ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
        auto dummy = new ListNode(-1), tail = dummy;
        while (l1 && l2) {
            if (l1->val > l2->val) {
                tail = tail->next = l2;
                l2 = l2->next;
            }
            else {
                tail = tail->next = l1;
                l1 = l1->next;
            }
        }
        // if (l1) tail = tail->next = l1;
        // if (l2) tail = tail->next = l2;
        // 上面两行代码可以写成如下一行
        tail = tail->next = (l1 != NULL ? l1 : l2);
        return dummy->next;
    }
};