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.

含义：合并另个已经排好序的列表

 递归方式

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

 非递归方式

private static ListNode mergeList(ListNode head1, ListNode head2) {
        ListNode newHead = new ListNode(0);
        ListNode newTail = newHead;
        while (head1 != null && head2 != null) {
            ListNode newNode;
            if (head1.value < head2.value) {
                newNode = new ListNode(head1.value);
                head1 = head1.next;
            } else {
                newNode = new ListNode(head2.value);
                head2 = head2.next;
            }
            newTail.next = newNode;
            newTail = newNode;
        }

        if (head1 != null) {
            newTail.next = head1;
        }
        if (head2 != null) {
            newTail.next = head2;
        }
        return newHead.next;
    }