剑指 Offer 25. 合并两个排序的链表

/**
 * 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) {
        ListNode p = new ListNode(-1), pre = p;
        while (l1 != null && l2 != null)
        {
            if(l1.val <= l2.val)
            {
                pre.next = l1;
                pre = pre.next;
                l1 = l1.next;
            }else{
                pre.next = l2;
                pre = pre.next;
                l2 = l2.next;
            }
        }
        if (l1 != null)
        {
            pre.next = l1;
        }
        if (l2 != null)
        {
            pre.next = l2;
        }
        return p.next;

    }
}