LeetCode21. 合并两个有序链表

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：

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() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        ListNode ls = new ListNode(0);
        ListNode shuai = ls;
        while(l1 != null && l2 != null){
            if(l1.val <= l2.val){
                shuai.next = l1;
                l1 = l1.next;
            }else{
                shuai.next = l2;
                l2 = l2.next;
            }
            shuai = shuai.next;
        }
        ListNode o = l1 != null ? l1 : l2;
        shuai.next = o;

        return ls.next;
    }
}