21. 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) {
        ListNode *mirror,*pre;
        mirror = new ListNode;
        pre = mirror;
        while(l1&&l2)
        {
            if(l1->val<l2->val)
            {
                pre->next = l1;//1->1
                l1 = l1->next;//p1 = 2
            }
            else
            {
                pre->next = l2;//0->1
                l2 = l2->next;//p2 = 3
            }
            pre = pre->next;//pre = 1
        }
        pre->next = l1 == NULL ? l2 : l1;
        return mirror->next;
        delete(mirror);
        
    }
};