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.

使用一个dummy node，对于要改变head node的链表，使用dummy node很有用。

public class Solution {
    //Time: O(n)  Space: O(1)
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        ListNode dummy = new ListNode(0);
        ListNode head = dummy;
        while (l1 != null || l2 != null) {
            if (l1 == null) {
                head.next = l2;
                l2 = l2.next;
            } else if (l2 == null) {
                head.next = l1;
                l1 = l1.next;
            } else {
                if (l1.val > l2.val) {
                    head.next = l2;
                    l2 = l2.next;
                } else {
                    head.next = l1;
                    l1 = l1.next;
                }
            }
            head = head.next;
        }
        return dummy.next;
    }
}