leetcode -- 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.

/**
 * 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) {
        // Start typing your Java solution below
        // DO NOT write main() function
        if(l1 == null){
            return l2;
        }
        if(l2 == null){
            return l1;
        }
        ListNode head = new ListNode(-1);
        ListNode tmp = head;
        while(l1 != null && l2 != null){
            if(l1.val < l2.val){
                tmp.next = l1;
                l1 = l1.next;
            } else {
                tmp.next = l2;
                l2 = l2.next;
            }
            tmp = tmp.next;
        }
        
        if(l1 != null){
            tmp.next = l1;
        }
        if(l2 != null){
            tmp.next = l2;
        }
        return head.next;
    }
}