21. 合并两个有序链表

21. 合并两个有序链表

来自 <https://leetcode.cn/problems/merge-two-sorted-lists/> 

/**
 * 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* list1, ListNode* list2) {
        ListNode * dummy=new ListNode(-1);
        ListNode* ptr=dummy;
        if(list1==nullptr){
            return list2;
        }
        if(list2==nullptr){
            return list1;
        }
        while(list1!=nullptr && list2!=nullptr){
            if(list1->val<list2->val){
                ptr->next=list1;
                // ptr=ptr->next;
                list1=list1->next;
            }else{
                ptr->next=list2;
                // ptr=ptr->next;
                list2=list2->next;
            }
            ptr=ptr->next;
        }
        // 扫尾
        if(list1!=nullptr){
            ptr->next=list1;
           
        }
        if(list2!=nullptr){
            ptr->next=list2;
           
        }
        return dummy->next;
    }
};

![image](https://img2022.cnblogs.com/blog/2269816/202210/2269816-20221016183636982-918698286.png)