LeetCode 21.合并两个有序链表_listnode(): val(0), next(nullptr){} listnode(int x

每次比较两个首节点，取小，循环比较拿取即可

/*
* 思路：两个链表首节点比较，取小者，循环比较插入即可
*/
 
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* head = nullptr, * tail = nullptr;
        head = tail = new ListNode();
        while (list1 && list2)
        {
            if (list1->val < list2->val)
            {
                tail->next = list1;
                list1 = list1->next;
            }
            else{
                tail->next = list2;
                list2 = list2->next;
            }
            tail = tail->next;
        }
        if (!list1)
            tail->next = list2;
        if (!list2)
            tail->next = list1;
        return head->next;
    }
};