21. Merge Two Sorted Lists Java solutions

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.

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