1.

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        ListNode temp=new ListNode(0);
        ListNode firstNode=temp;
        while(l1!=null && l2!=null){
            if(l1.val>l2.val){
                temp.next=l2;
                l2=l2.next;
                temp=temp.next;
            }
            else{
                temp.next=l1;
                l1=l1.next;
                temp=temp.next;
            }
        }
        //說明l1或者l2為空
        while(l1!=null){
            temp.next=l1;
            l1=l1.next;
            temp=temp.next;
        }
        while(l2!=null){
            temp.next=l2;
            l2=l2.next;
            temp=temp.next;
        }
        return firstNode.next;
    }
}
 

2.

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode mergeKLists(ListNode[] lists) {
		if (lists == null || lists.length == 0)
			return null;
		return MSort(lists, 0, lists.length - 1);
	}
 
	public ListNode MSort(ListNode[] lists, int low, int high) {
		if (low < high) {
			int mid = (low + high) / 2;
			ListNode leftlist = MSort(lists, low, mid);
			ListNode rightlist = MSort(lists, mid + 1, high);
			return mergeTwoLists(leftlist, rightlist);
		}
		// 如果相等，只有一個元素，返回即可
		return lists[low];
	}
 
	// 遞迴合併連結串列
	public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
		ListNode res = null;
		if (l1 == null)
			return l2;
		if (l2 == null)
			return l1;
		if (l1.val <= l2.val) {
			res = l1;
			l1.next = mergeTwoLists(l1.next, l2);
		} else {
			res = l2;
			l2.next = mergeTwoLists(l1, l2.next);
		}
		return res;
	}

}