題目：

Merge two sorted linked lists and return it as a new list. The new list should be made by splicing together the nodes of the first two lists.

演算法如下：

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
 

public class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        ListNode head = null;
        if(l1==null&&l2==null){
            return null;
        }
        if(l1==null){
            return l2;
        }
        if(l2==null){
            return l1;
        }
        if
 
(l1.val<l2.val){
            head = l1;
            l1 = l1.next;
        }else{
            head = l2;
            l2 = l2.next;
        }
        ListNode newHead = head;
        while(l1!=null&&l2!=null){
            while(l1.val<l2.val){
                head.next = l1;
                head = head.next;
                l1 = l1.next;
                if
 
(l1==null||l2==null) break;
            }
            if(l1==null||l2==null) break;
            while(l1.val>=l2.val){
                head.next = l2;
                head = head.next;
                l2 = l2.next;
                if(l2==null||l1==null) break;
            }
        }
        while(l1!=null){
            head.next = l1;
            head = head.next;
            l1 = l1.next;
        }
        while(l2!=null){
            head.next = l2;
            head = head.next;
            l2 = l2.next;
        }

        return newHead;
    }
}