第一天
递归:LeetCode21.合并两个有序链表
/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* ListNode *next;
* ListNode() : val(0), next(nullptr) {}
* ListNode(int x) : val(x), next(nullptr) {}
* ListNode(int x, ListNode *next) : val(x), next(next) {}
* };
*/
class Solution {
public:
ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
if (!l1)
return l2;
if (!l2)
return l1;
if (l1->val < l2->val) {
l1->next = mergeTwoLists(l1->next, l2);
return l1;