Merge Two Sorted Lists

public class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        ListNode safe = new ListNode(-1);
        ListNode pre = safe;
        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 safe.next;
    }
}