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 *dummyNode = new ListNode(0);
        ListNode* prev = dummyNode;
        while(l1!=nullptr && l2!=nullptr)
        {
            if(l1->val < l2->val)
            {
                prev->next = l1;
                l1 = l1->next;
            }
            else
            {
                prev->next = l2;
                l2 = l2->next;
            }
            prev = prev->next;
        }

        if(l1 != nullptr)
        {
            prev->next = l1;
        }

        else if(l2 != nullptr)
         {
            prev->next = l2;
         }

         return dummyNode->next;
    }
};