合并两个有序链表，并将结果作为一个新的链表返回。应该通过将前两个列表的节点拼接在一起来创建新列表。

举例：
输入： 1->2->4, 1->3->4
输出： 1->1->2->3->4->4

/*
 * 21. Merge Two Sorted Lists
 * https://leetcode.com/problems/merge-two-sorted-lists/
 * https://www.whosneo.com/21-merge-two-sorted-lists/
 */

public class MergeTwoLists {
    public static void main(String[] args) {
        ListNode head1 = new ListNode(1);
        head1.next = new ListNode(2);
        head1.next.next = new ListNode(4);
        ListNode head2 = new ListNode(1);
        head2.next = new ListNode(3);
        head2.next.next = new ListNode(4);

        MergeTwoLists solution = new MergeTwoLists();

        solution.print(head1);
        solution.print(head2);
        ListNode node = solution.mergeTwoLists(head1, head2);
        solution.print(node);
    }

    private void print(ListNode node) {
        for (; node != null; node = node.next) {
            System.out.print(node.val);
            System.out.print("->");
        }
        System.out.println("null");
    }

    private ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        ListNode prev = new ListNode(0);
        ListNode node = prev;
        while (l1 != null && l2 != null) {
            if (l1.val < l2.val) {
                node.next = l1;
                l1 = l1.next;
            } else {
                node.next = l2;
                l2 = l2.next;
            }
            node = node.next;
        }
        node.next = l1 != null ? l1 : l2;
        return prev.next;
    }
}