21 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.

public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
    if (l1 == null && l2 == null) {
        return null;
    } else if (l1 == null && l2 != null) {
        return l2;
    } else if (l1 != null && l2 == null) {
        return l1;
    }

    ListNode dummy = new ListNode(-1);
    ListNode cur = dummy;
    while (l1 != null && l2 != null) {
        if (l1.val > l2.val) {
            cur.next = l2;
            l2 = l2.next;
        } else {
            cur.next = l1;
            l1 = l1.next;
        }
        cur = cur.next;
    }

    // 因为是linkedList所以不用while来loop直接把多了的贴上就ok了
    if (l1 != null) {
        cur.next = l1;
    }

    if (l2 != null) {
        cur.next = l2;
    }

    return dummy.next;
}