Merge Two Sorted Lists

Merge two sorted (ascending) linked lists and return it as a new sorted list. The new sorted list should be made by splicing together the nodes of the two lists and sorted in ascending order.

Example

Given1->3->8->11->15->null,2->null, return1->2->3->8->11->15->null.

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