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.

	// 非遞迴實現
	public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
		ListNode head = new ListNode(0);
		ListNode cur = head;
		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;
		}

		if (l1 != null)
			cur.next = l1;
		if (l2 != null)
			cur.next = l2;
		return head.next;
	}

	// 遞迴實現
	public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
		ListNode res = null;
		if (l1 == null)
			return l2;
		if (l2 == null)
			return l1;
		if (l1.val <= l2.val) {
			res = l1;
			l1.next = mergeTwoLists(l1.next, l2);
		} else {
			res = l2;
			l2.next = mergeTwoLists(l1, l2.next);
		}
		return res;
	}