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.

原题链接：https://oj.leetcode.com/problems/merge-two-sorted-lists/

题目：归并两个有序链表，返回一个新的有序链表。

思路：同归并数组的解法一样，依次比較首位元素。

	public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
		ListNode p1 = l1, p2 = l2, dummy = new ListNode(-1), p = dummy;
		while (p1 != null && p2 != null) {
			if (p1.val <= p2.val) {
				p.next = p1;
				p1 = p1.next;
			} else {
				p.next = p2;
				p2 = p2.next;
			}
			p = p.next;
		}
		if (p1 != null)
			p.next = p1;
		if (p2 != null)
			p.next = p2;
		return dummy.next;
	}