将两个有序链表合并为一个新的有序链表并返回。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：

输入：1->2->4, 1->3->4
输出：1->1->2->3->4->4

/**
 * 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) {
        if(l1==null){
            return l2;
        }
        
        if(l2==null) {
            return l1;
        }
        
        ListNode newHead = null;
        ListNode tempL = null;
        while(l1!=null && l2!=null) {
            if(l1.val<=l2.val) {
                if(newHead==null) {
                    newHead = l1;
                }
                
                if(tempL==null) {
                    tempL = l1;
                } else {
                    tempL.next = l1;
                    tempL = l1;
                }
                
                l1 = l1.next;
            } else {
                if(newHead==null) {
                    newHead = l2;
                }
                
                if(tempL==null) {
                    tempL = l2;
                } else {
                    tempL.next = l2;
                    tempL = l2;
                }
                
                l2 = l2.next;
            }
        }
        
        if(l1!=null) {
            tempL.next = l1;
        }
        
        if(l2!=null) {
            tempL.next = l2;
        }
        
        return newHead;
    }
}