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.

是在l1的基础上连接的, 所以要注意需要链接l1

l2.next = l1;

需要不断更新前一个节点
pre = pre.next;

public class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        if(l1 == null) return l2;
        if(l2 == null) return l1;
        ListNode dummy = new ListNode(0);
        dummy.next = l1;
        ListNode pre = dummy;
        while (l1 != null && l2 != null) {
            if (l1.val <= l2.val) {
                l1 = l1.next;
            } else {
                ListNode next = l2.next;
                //ListNode temp = pre.next;
                pre.next = l2;
                l2.next = l1;
                l2 = next;
            }
            pre = pre.next;
        }
        if (l2 != null) {
            pre.next = l2;
        }
        return dummy.next;
    }
}