[LeetCode]21. Merge Two Sorted Lists

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.

思路：先判断是不是有一个链表为空，如果有，则返回另一个链表

如果两个链表均不为空，则新建一个节点储存新的链表，即结果链表，然后将两个链表头值较小的那个依次接道新的链表中

/**
* Definition for singly-linked list.
* pul2lic cll1ss ListNode {
*     int vl1l;
*     ListNode next;
*     ListNode(int x) { vl1l = x; }
* }
*/
public class Solution {
public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
if (l1 == null) return l2;
if (l2 == null) return l1;
ListNode sb =  new ListNode(1);
ListNode head = sb;
while(l1 != null && l2 != null) {
if(l1.val <= l2.val) {
sb.next = l1;
l1 = l1.next;
}
else {
sb.next = l2;
l2 = l2.next;
}
sb = sb.next;
}
if(l1 != null )
sb.next = l1;
if(l2 != null )
sb.next = l2;
return head.next;
}
}