将两个升序链表合并为一个新的 升序 链表并返回。新链表是通过拼接给定的两个链表的所有节点组成的。

示例 1：

输入：l1 = [1,2,4], l2 = [1,3,4]
输出：[1,1,2,3,4,4]

示例 2：

输入：l1 = [], l2 = []
输出：[]

示例 3：

输入：l1 = [], l2 = [0]
输出：[0]

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode mergeTwoLists(ListNode list1, ListNode list2) {
        ListNode dume = new ListNode();
        ListNode result = dume;
        ListNode first = list1;
        ListNode second = list2;
        while (first != null && second !=null ){
            if(first.val < second.val){
                ListNode temp = new ListNode();
                temp.val=first.val;
                dume.next = temp;
                dume= dume.next;
                first = first.next;
            
            }
            else if (first.val == second.val){
                ListNode temp = new ListNode();
                temp.val = first.val;
                dume.next = temp;
                dume = dume.next;
                ListNode tempp = new ListNode();
                tempp.val = second.val;
                dume.next = tempp;
                dume = dume.next;
                first = first.next;
                second = second.next;
            
            }
            else if (first.val > second.val){
                ListNode temp = new ListNode();
                temp.val = second.val;
                dume.next = temp;
                dume= dume.next;
                second = second.next;
         
            }
        }
        if(first != null && second == null){
            dume.next=first;
        }
        if(first == null && second != null){
            dume.next=second;
        }
        return result.next;
    }
}