合并两个有序链表（c++/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* list1, ListNode* list2) {
        if(!list1){
            return list2;
        }
        if(!list2){
            return list1;
        }
        ListNode* Listhead = new ListNode(-1);
        ListNode* List = Listhead;
        while(list1 && list2){
            if(list1->val < list2->val){
                List = List->next = list1;
                list1 = list1->next;
            }else{
                List = List->next = list2;
                list2 = list2->next;
            }
        }
        if(list1){
            List->next = list1;
        }
        if(list2){
            List->next = list2;
        }  
        return Listhead->next;    
    }
};

/**
 * 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) {
        if(!list1){
            return list2;
        }else if(!list2){
            return list1;
        }else if(list1->val < list2->val){
            list1->next = mergeTwoLists(list1->next,list2);
            return list1;
        }else{
            list2->next = mergeTwoLists(list1,list2->next);
            return list2;
        }
    }
};