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.
Solution:
时间复杂度是O(m+n),m和n分别是两条链表的长度，空间复杂度是O(1)

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        ListNode helper = new ListNode(0);
        ListNode pre = helper;
        helper.next = l1;
        while(l1 != null && l2 != null) {
            if(l1.val > l2.val) {
                ListNode next = l2.next;
                l2.next = pre.next;
                pre.next = l2;
                l2 = next;
            }else {
                l1 = l1.next;
            }
            pre = pre.next;
        }
        if(l2 != null) {
            pre.next = l2;
        }
        return helper.next;
    }
}